z3-src 503.2.0

/*++
Copyright (c) 2023 Microsoft Corporation

Module Name:

    euf_ac_plugin.cpp

Abstract:

    plugin structure for ac functions

Author:

    Nikolaj Bjorner (nbjorner) 2023-11-11

Completion modulo AC

  E set of eqs
  pick critical pair xy = z by j1 xu = v by j2 in E
  Add new equation zu = xyu = vy by j1, j2


  Sets P - processed, R - reductions, S - to simplify

  new equality l = r:
      reduce l = r modulo R if equation is external
      orient l = r - if it cannot be oriented, discard
      if l = r is a reduction rule then reduce R, S using l = r, insert into R
      else insert into S

  main loop:
  for e as (l = r) in S:
      remove e from S
      backward simplify e
      if e is backward subsumed, continue
      if e is a reduction rule, then reduce R, S using e, insert into R, continue
      insert e into P
      superpose with e
      forward simplify with e

backward simplify e as (l = r) using (l' = r') in P u S:
  if l' is a subset or r then replace l' by r' in r.

backward subsumption e as (l = r) using (l' = r') in P u S:
  l = r is of the form l'x = r'x

is reduction rule e as (l = r):
  l is a unit, and r is unit, is empty, or is zero.

superpose e as (l = r) with (l' = r') in P:
  if l and l' share a common subset x.

forward simplify (l' = r') in P u S using e as (l = r):


More notes:

  Justifications for new equations are joined (requires extension to egraph/justification)

  Process new merges so use list is updated
  Justifications for processed merges are recorded

  Updated equations are recorded for restoration on backtracking

  Keep track of foreign / shared occurrences of AC functions.
  - use register_shared to accumulate shared occurrences.

  Shared occurrences are rewritten modulo completion.
  When equal to a different shared occurrence, propagate equality.

  - Elimination of redundant rules.
  -> forward and backward subsumption
     - apply forward subsumption when simplifying equality using processed
     - apply backward subsumption when simplifying processed and to_simplify

   Rewrite rules are reoriented after a merge of enodes.
   It simulates creating a critical pair:
   n -> n'
   n + k = j + k
   after merge
   n' + k = j + k, could be that n' + k < j + k < n + k in term ordering because n' < j, m < n

  We filter results of superposition so that the size of monomials in the new equations don't grow.
  This filter is used to make the process somewhat tractable. In other words, we never compute a
  complete saturation. If l1 = r1 l2 = r2 are used to produce new equation l = r, we ensure
  that min(|l|,|r|) <= min(|r1|,|r2|) and max(|l|,|r|) <= max(|l1|,|l2|)

  If the operator is injective we simplify equations
  xl = xr to l = r

TODOs:

- Efficiency of handling shared terms.
  - The shared terms hash table is not incremental.
    It could be made incremental by updating it on every merge similar to how the egraph handles it.
- V2 using multiplicities instead of repeated values in monomials.
- Squash trail updates when equations or monomials are modified within the same epoch.
  - by an epoch counter that can be updated by the egraph class whenever there is a push/pop.
  - store the epoch as a tick on equations and possibly when updating monomials on equations.


--*/

#include "ast/euf/euf_ac_plugin.h"
#include "ast/euf/euf_egraph.h"
#include "ast/ast_pp.h"

namespace euf {

    ac_plugin::ac_plugin(egraph& g, char const* name, unsigned fid, unsigned op) :
        plugin(g), m_fid(fid), m_op(op),
        m_dep_manager(get_region()),
        m_hash(*this), m_eq(*this), m_monomial_table(m_hash, m_eq)
    {
        g.set_th_propagates_diseqs(m_fid);
        m_name = symbol(name);
    }

    ac_plugin::ac_plugin(egraph& g, func_decl* f) :
        plugin(g), m_fid(f->get_family_id()), m_decl(f),
        m_dep_manager(get_region()),
        m_hash(*this), m_eq(*this), m_monomial_table(m_hash, m_eq)
    {
        m_name = f->get_name();
        if (m_fid != null_family_id)
            g.set_th_propagates_diseqs(m_fid);
    }

    void ac_plugin::register_node(enode* n) {
        if (is_op(n))
            return;
        for (auto arg : enode_args(n))
            if (is_op(arg))
                register_shared(arg);
    }

    // unit -> {}
    void ac_plugin::add_unit(enode* u) {
        push_equation(u, nullptr);
    }

    // zero x -> zero
    void ac_plugin::add_zero(enode* z) {
        mk_node(z)->is_zero = true;
        // zeros persist
    }

    void ac_plugin::register_shared(enode* n) {
        if (m_shared_nodes.get(n->get_id(), false))
            return;
        auto m = to_monomial(n);
        auto const& ns = monomial(m);
        auto idx = m_shared.size();
        for (auto arg : ns) {
            arg->shared.push_back(idx);
            m_node_trail.push_back(arg);
            push_undo(undo_kind::is_add_shared_index);
        }
        m_shared_nodes.setx(n->get_id(), true, false);
        sort(monomial(m));
        m_shared_todo.insert(idx);
        m_shared.push_back({ n, m, justification::axiom(get_id()) });
        push_undo(undo_kind::is_register_shared);
    }

    void ac_plugin::push_scope_eh() {
        push_undo(undo_kind::is_push_scope);
    }

    void ac_plugin::undo() {
        auto k = m_undo.back();
        m_undo.pop_back();
        switch (k) {
        case undo_kind::is_queue_eq: {
            m_queued.pop_back();
            break;
        }
        case undo_kind::is_add_node: {
            auto* n = m_node_trail.back();
            m_node_trail.pop_back();
            m_nodes[n->n->get_id()] = nullptr;
            n->~node();
            break;
        }
        case undo_kind::is_push_scope: {
            m_active.reset();
            m_passive.reset();
            m_units.reset();
            m_queue_head = 0;
            break;
        }
        case undo_kind::is_add_monomial: {
            m_monomials.pop_back();
            break;
        }
        case undo_kind::is_add_shared_index: {
            auto n = m_node_trail.back();
            m_node_trail.pop_back();
            n->shared.pop_back();
            break;
        }
        case undo_kind::is_add_eq_index: {
            auto n = m_node_trail.back();
            m_node_trail.pop_back();
            n->eqs.pop_back();
            break;
        }
        case undo_kind::is_register_shared: {
            auto s = m_shared.back();
            m_shared_nodes[s.n->get_id()] = false;
            m_shared.pop_back();
            break;
        }
        case undo_kind::is_update_shared: {
            auto [id, s] = m_update_shared_trail.back();
            m_shared[id] = s;
            m_update_shared_trail.pop_back();
            break;
        }
        }
    }

    std::ostream& ac_plugin::display_monomial(std::ostream& out, ptr_vector<node> const& m) const {
        for (auto n : m) {
            if (n->n->num_args() == 0)
                out << n->n->get_expr_id() << ": " << mk_pp(n->n->get_expr(), g.get_manager()) << " ";
            else
                out << g.bpp(n->n) << " ";
        }
        return out;
    }

    std::ostream& ac_plugin::display_equation(std::ostream& out, eq const& e) const {
        display_status(out, e.status) << " ";
        display_monomial(out, monomial(e.l));
        out << "== ";
        display_monomial(out, monomial(e.r));
        return out;
    }

    std::ostream& ac_plugin::display_monomial_ll(std::ostream& out, ptr_vector<node> const& m) const {
        for (auto n : m)
            out << n->id() << " ";
        return out;
    }

    std::ostream& ac_plugin::display_equation_ll(std::ostream& out, eq const& e) const {
        display_status(out, e.status) << " ";
        display_monomial_ll(out, monomial(e.l));
        out << "== ";
        display_monomial_ll(out, monomial(e.r));
        return out;
    }

    std::ostream& ac_plugin::display_status(std::ostream& out, eq_status s) const {
        switch (s) {
        case eq_status::is_dead_eq: out << "d"; break;
        case eq_status::is_processed_eq: out << "p"; break;
        case eq_status::is_to_simplify_eq: out << "s"; break;
        case eq_status::is_reducing_eq: out << "r"; break;
        case eq_status::is_passive_eq: out << "x"; break;
        }
        return out;
    }

    std::ostream& ac_plugin::display(std::ostream& out) const {
        out << m_name << "\n";
        unsigned i = 0;
        for (auto const& eq : m_active) {
            if (eq.status != eq_status::is_dead_eq) {
                out << "["; display_status(out, eq.status) << "] " << i << " : " << eq_pp_ll(*this, eq) << "\n";
            }
            ++i;
        }

        if (!m_shared.empty())
            out << "shared monomials:\n";
        for (auto const& s : m_shared) {
            out << g.bpp(s.n) << " r " << g.bpp(s.n->get_root()) << " - " << s.m << ": " << m_pp_ll(*this, monomial(s.m)) << "\n";
        }

        for (auto n : m_nodes) {
            if (!n)
                continue;
            if (n->eqs.empty() && n->shared.empty())
                continue;
            out << g.bpp(n->n) << " ";
            if (!n->eqs.empty()) {
                out << "eqs ";
                for (auto l : n->eqs)
                    out << l << " ";
            }
            if (!n->shared.empty()) {
                out << "shared ";
                for (auto s : n->shared)
                    out << s << " ";
            }
            out << "\n";
        }
        return out;
    }

    void ac_plugin::collect_statistics(statistics& st) const {
        std::string name = m_name.str();
        m_superposition_stats = symbol((std::string("ac ") + name + " superpositions"));
        m_eqs_stats = symbol((std::string("ac ") + name + " equations"));
        st.update(m_superposition_stats.bare_str(), m_stats.m_num_superpositions);
        st.update(m_eqs_stats.bare_str(), m_active.size());
    }

    void ac_plugin::merge_eh(enode* l, enode* r) {
        push_equation(l, r);
    }

    void ac_plugin::pop_equation(enode* l, enode* r) {
        m_fuel += m_fuel_inc;
        if (!r) {
            m_units.push_back(l);
            mk_node(l);
            auto m_id = to_monomial(l, {});
            init_equation(eq(to_monomial(l), m_id, justification::axiom(get_id())), true);
        }
        else {
            auto j = justification::equality(l, r);
            auto m1 = to_monomial(l);
            auto m2 = to_monomial(r);
            TRACE(plugin, tout << "merge: " << m_name << " " << g.bpp(l) << " == " << g.bpp(r) << " " << m_pp_ll(*this, monomial(m1)) << " == " << m_pp_ll(*this, monomial(m2)) << "\n");
            init_equation(eq(m1, m2, j), true);
        }
    }

    void ac_plugin::diseq_eh(enode* eq) {
        SASSERT(g.get_manager().is_eq(eq->get_expr()));
        enode* a = eq->get_arg(0), * b = eq->get_arg(1);
        a = a->get_closest_th_node(m_fid);
        b = b->get_closest_th_node(m_fid);
        SASSERT(a && b);
        register_shared(a);
        register_shared(b);
    }

    void ac_plugin::push_equation(enode* l, enode* r) {
        if (l == r)
            return;
        m_queued.push_back({ l, r });
        push_undo(undo_kind::is_queue_eq);
    }

    bool ac_plugin::init_equation(eq eq, bool is_active) {
        deduplicate(monomial(eq.l), monomial(eq.r));
        if (!orient_equation(eq))
            return false;

        if (is_reducing(eq))
            is_active = true;

        if (!is_active) {
            m_passive.push_back(eq);
            return true;          
        }

        eq.status = eq_status::is_to_simplify_eq;

        m_active.push_back(eq);
        auto& ml = monomial(eq.l);
        auto& mr = monomial(eq.r);

        unsigned eq_id = m_active.size() - 1;

        if (ml.size() == 1 && mr.size() == 1)
            push_merge(ml[0]->n, mr[0]->n, eq.j);

        for (auto n : ml) {
            if (!n->n->is_marked2()) {
                n->eqs.push_back(eq_id);
                n->n->mark2();
                push_undo(undo_kind::is_add_eq_index);
                m_node_trail.push_back(n);
                for (auto s : n->shared)
                    m_shared_todo.insert(s);
            }
        }

        for (auto n : mr) {
            if (!n->n->is_marked2()) {
                n->eqs.push_back(eq_id);
                n->n->mark2();
                push_undo(undo_kind::is_add_eq_index);
                m_node_trail.push_back(n);
                for (auto s : n->shared)
                    m_shared_todo.insert(s);
            }
        }

        for (auto n : ml)
            n->n->unmark2();

        for (auto n : mr)
            n->n->unmark2();

        SASSERT(well_formed(eq));

        TRACE(plugin, display_equation_ll(tout, eq) << " shared: " << m_shared_todo << "\n");
        m_to_simplify_todo.insert(eq_id);
        m_new_eqs.push_back(eq_id);

        //display_equation_ll(verbose_stream() << "init " << eq_id << ": ", eq) << "\n";

        return true;
    }
    

    bool ac_plugin::orient_equation(eq& e) {
        auto& ml = monomial(e.l);
        auto& mr = monomial(e.r);
        if (ml.size() > mr.size())
            return true;
        if (ml.size() < mr.size()) {
            std::swap(e.l, e.r);
            return true;
        }
        else {
            sort(ml);
            sort(mr);
            for (unsigned i = 0; i < ml.size(); ++i) {
                if (ml[i]->id() == mr[i]->id())
                    continue;
                if (ml[i]->id() < mr[i]->id())
                    std::swap(e.l, e.r);
                return true;
            }
            return false;
        }
    }

    bool ac_plugin::is_equation_oriented(eq const& e) const {
        auto& ml = monomial(e.l);
        auto& mr = monomial(e.r);
        if (ml.size() > mr.size())
            return true;
        if (ml.size() < mr.size())
            return false;
        else {
            if (!is_sorted(ml))
                return false;
            if (!is_sorted(mr))
                return false;
            for (unsigned i = 0; i < ml.size(); ++i) {
                if (ml[i]->id() == mr[i]->id())
                    continue;
                if (ml[i]->id() < mr[i]->id())
                    return false;
                return true;
            }
            return false;
        }
    }

    void ac_plugin::sort(monomial_t& m) {
        std::sort(m.begin(), m.end(), [&](node* a, node* b) { return a->id() < b->id(); });
    }

    bool ac_plugin::is_sorted(monomial_t const& m) const {
        if (m.m_bloom.m_tick == m_tick)
            return true;
        for (unsigned i = m.size(); i-- > 1; )
            if (m[i - 1]->id() > m[i]->id())
                return false;
        return true;
    }

    uint64_t ac_plugin::filter(monomial_t& m) {
        auto& bloom = m.m_bloom;

        if (bloom.m_tick == m_tick) {
            SASSERT(bloom_filter_is_correct(m.m_nodes, m.m_bloom));
            return bloom.m_filter;
        }
        bloom.m_filter = 0;
        for (auto n : m)
            bloom.m_filter |= (1ull << (n->id() % 64ull));
        if (!is_sorted(m))
            sort(m);
        bloom.m_tick = m_tick;
        return bloom.m_filter;
    }

    bool ac_plugin::bloom_filter_is_correct(ptr_vector<node> const& m, bloom const& b) const {
        uint64_t f = 0;
        for (auto n : m)
            f |= (1ull << (n->id() % 64ull));
        return b.m_filter == f;
    }

    bool ac_plugin::can_be_subset(monomial_t& subset, monomial_t& superset) {
        if (subset.size() > superset.size())
            return false;
        auto f1 = filter(subset);
        auto f2 = filter(superset);
        return (f1 | f2) == f2;
    }

    bool ac_plugin::can_be_subset(monomial_t& subset, ptr_vector<node> const& m, bloom& bloom) {
        if (subset.size() > m.size())
            return false;
        SASSERT(bloom.m_tick != m_tick || bloom_filter_is_correct(m, bloom));
        if (bloom.m_tick != m_tick) {
            bloom.m_filter = 0;
            for (auto n : m)
                bloom.m_filter |= (1ull << (n->id() % 64ull));
            bloom.m_tick = m_tick;
        }
        auto f2 = bloom.m_filter;
        return (filter(subset) | f2) == f2;
    }


    void ac_plugin::push_undo(undo_kind k) {
        m_undo.push_back(k);
        push_plugin_undo(get_id());
        m_undo_notify(); // tell main plugin to dispatch undo to this module.
    }

    unsigned ac_plugin::to_monomial(enode* n) {
        enode_vector& ns = m_todo;
        ns.reset();
        ptr_vector<node> m;
        ns.push_back(n);
        for (unsigned i = 0; i < ns.size(); ++i) {
            auto k = ns[i];
            if (is_op(k))
                ns.append(k->num_args(), k->args());
            else
                m.push_back(mk_node(k));
        }
        return to_monomial(n, m);
    }

    unsigned ac_plugin::to_monomial(enode* e, ptr_vector<node> const& ms) {
        unsigned id = m_monomials.size();
        m_monomials.push_back({ ms, bloom(), e });
        push_undo(undo_kind::is_add_monomial);
        return id;
    }

    enode* ac_plugin::from_monomial(ptr_vector<node> const& mon) {
        auto& m = g.get_manager();
        ptr_buffer<expr> args;
        enode_vector nodes;
        for (auto arg : mon) {
            nodes.push_back(arg->n);
            args.push_back(arg->n->get_expr());
        }
        expr* n = nullptr;
        switch (args.size()) {
        case 0:
            UNREACHABLE();
            break;
        case 1:
            n = args[0];
            break;
        default:
            n = m.mk_app(m_fid, m_op, args.size(), args.data());
            break;
        }
        auto r = g.find(n);
        return r ? r : g.mk(n, 0, nodes.size(), nodes.data());
    }

    ac_plugin::node* ac_plugin::node::mk(region& r, enode* n) {
        auto* mem = r.allocate(sizeof(node));
        node* res = new (mem) node();
        res->n = n;
        return res;
    }

    ac_plugin::node* ac_plugin::mk_node(enode* n) {
        unsigned id = n->get_id();
        if (m_nodes.size() > id && m_nodes[id])
            return m_nodes[id];
        auto* r = node::mk(get_region(), n);
        push_undo(undo_kind::is_add_node);
        m_nodes.setx(id, r, nullptr);
        m_node_trail.push_back(r);
        if (is_op(n)) {
            // extract shared sub-expressions
        }
        return r;
    }

    void ac_plugin::propagate() {
        while (m_queue_head < m_queued.size()) {
            auto [l, r] = m_queued[m_queue_head++];
            pop_equation(l, r);
        }
        while (true) {
        loop_start:
            if (m_fuel == 0)
                break;
            unsigned eq_id = pick_next_eq();
            if (eq_id == UINT_MAX)
                break;

            TRACE(plugin, tout << "propagate " << eq_id << ": " << eq_pp_ll(*this, m_active[eq_id]) << "\n");

            SASSERT(well_formed(m_active[eq_id]));

            // simplify eq using processed
            TRACE(plugin,
                for (auto other_eq : forward_iterator(eq_id))
                    tout << "forward iterator " << eq_pp_ll(*this, m_active[eq_id]) << " vs " << eq_pp_ll(*this, m_active[other_eq])  << "\n");
            for (auto other_eq : forward_iterator(eq_id))
                if ((is_processed(other_eq) || is_reducing(other_eq)) && forward_simplify(eq_id, other_eq))
                    goto loop_start;

            auto& eq = m_active[eq_id];
            deduplicate(monomial(eq.l), monomial(eq.r));
            if (monomial(eq.l).size() == 0) {
                set_status(eq_id, eq_status::is_dead_eq);
                continue;
            }
            if (is_forward_subsumed(eq_id)) {
                set_status(eq_id, eq_status::is_dead_eq);
                continue;
            }

            if (is_reducing(eq)) {
                set_status(eq_id, eq_status::is_reducing_eq);
                backward_reduce(eq_id);
                continue;
            }
            --m_fuel;
            set_status(eq_id, eq_status::is_processed_eq);

            // simplify processed using eq
            for (auto other_eq : backward_iterator(eq_id))
                if (is_active(other_eq))
                    backward_simplify(eq_id, other_eq);
            forward_subsume_new_eqs();
            
            // superpose, create new equations
            unsigned new_sup = 0;
            for (auto other_eq : superpose_iterator(eq_id))
                if (is_processed(other_eq))
                    new_sup += superpose(eq_id, other_eq);
            forward_subsume_new_eqs();

            m_stats.m_num_superpositions += new_sup;
            TRACE(plugin, tout << "new superpositions " << new_sup << "\n");
        }
        propagate_shared();

        CTRACE(plugin, !m_shared.empty() || !m_active.empty(), display(tout));
    }

    unsigned ac_plugin::pick_next_eq() {
        init_pick:
        while (!m_to_simplify_todo.empty()) {
            unsigned id = *m_to_simplify_todo.begin();
            if (id < m_active.size() && is_to_simplify(id))
                return id;
            m_to_simplify_todo.remove(id);
        }
        if (!m_passive.empty()) {
            auto eq = m_passive.back();
            // verbose_stream() << "pick passive " << eq_pp_ll(*this, eq) << "\n";
            m_passive.pop_back();
            init_equation(eq, true);
            goto init_pick;
        }
        return UINT_MAX;
    }

    // reorient equations when the status of equations are set to to_simplify.
    void ac_plugin::set_status(unsigned id, eq_status s) {
        auto& eq = m_active[id];
        if (eq.status == eq_status::is_dead_eq)
            return;
        if (are_equal(monomial(eq.l), monomial(eq.r)))
            s = eq_status::is_dead_eq;

        eq.status = s;
        switch (s) {
        case eq_status::is_processed_eq:
        case eq_status::is_reducing_eq:
        case eq_status::is_dead_eq:
        case eq_status::is_passive_eq:
            m_to_simplify_todo.remove(id);
            break;
        case eq_status::is_to_simplify_eq:
            m_to_simplify_todo.insert(id);
            if (!orient_equation(eq)) {
                set_status(id, eq_status::is_dead_eq);
            }
            break;
        }
    }

    //
    // superpose iterator enumerates all equations where lhs of eq have element in common.
    //  
    unsigned_vector const& ac_plugin::superpose_iterator(unsigned eq_id) {
        auto const& eq = m_active[eq_id];
        m_src_r.reset();
        m_src_r.append(monomial(eq.r).m_nodes);
        init_ref_counts(monomial(eq.l), m_src_l_counts);
        init_overlap_iterator(eq_id, monomial(eq.l));
        return m_eq_occurs;
    }

    //
    // forward iterator allows simplification of eq
    // The rhs of eq is a super-set of lhs of other eq.
    // 
    unsigned_vector const& ac_plugin::forward_iterator(unsigned eq_id) {
        auto const& eq = m_active[eq_id];
        init_ref_counts(monomial(eq.r), m_dst_r_counts);
        init_ref_counts(monomial(eq.l), m_dst_l_counts);
        if (monomial(eq.r).size() == 0) {
            m_dst_l.reset();
            m_dst_l.append(monomial(eq.l).m_nodes);
            init_subset_iterator(eq_id, monomial(eq.l));
            return m_eq_occurs;
        }
        m_dst_r.reset();
        m_dst_r.append(monomial(eq.r).m_nodes);
        init_subset_iterator(eq_id, monomial(eq.r));
        return m_eq_occurs;
    }

    void ac_plugin::init_overlap_iterator(unsigned eq_id, monomial_t const& m) {
        m_eq_occurs.reset();
        for (auto n : m)
            m_eq_occurs.append(n->eqs);
        compress_eq_occurs(eq_id);
    }

    //
    // add all but one of the use lists. Identify the largest use list and skip it.
    // The rationale is that [a, b] is a subset of [a, b, c, d, e] if
    // it has at least two elements (otherwise it would not apply as a rewrite over AC).
    // then one of the two elements has to be in the set of [a, b, c, d, e] \ { x }
    // where x is an arbitrary value from a, b, c, d, e. Not a two-element watch list, but still.
    //
    void ac_plugin::init_subset_iterator(unsigned eq_id, monomial_t const& m) {
        unsigned max_use = 0;
        node* max_n = nullptr;
        bool has_two = false;
        for (auto n : m)
            if (n->eqs.size() >= max_use)
                has_two |= max_n && (max_n != n), max_n = n, max_use = n->eqs.size();
        m_eq_occurs.reset();
        if (has_two) {
            for (auto n : m)
                if (n != max_n)
                    m_eq_occurs.append(n->eqs);
        }
        else {
            for (auto n : m) {
                m_eq_occurs.append(n->eqs);
                break;
            }
        }
        compress_eq_occurs(eq_id);
    }

    // prune m_eq_occurs to single occurrences
    void ac_plugin::compress_eq_occurs(unsigned eq_id) {
        unsigned j = 0;
        m_eq_seen.reserve(m_active.size() + 1, false);
        for (unsigned i = 0; i < m_eq_occurs.size(); ++i) {
            unsigned id = m_eq_occurs[i];
            if (m_eq_seen[id])
                continue;
            if (id == eq_id)
                continue;
            if (!is_active(id))
                continue;
            m_eq_occurs[j++] = id;
            m_eq_seen[id] = true;
        }
        m_eq_occurs.shrink(j);
        for (auto id : m_eq_occurs)
            m_eq_seen[id] = false;
    }

    //
    // backward iterator simplifies other eqs where their rhs is a superset of lhs of eq
    // 
    unsigned_vector const& ac_plugin::backward_iterator(unsigned eq_id) {
        auto& eq = m_active[eq_id];
        m_src_r.reset();
        m_src_r.append(monomial(eq.r).m_nodes);
        init_ref_counts(monomial(eq.l), m_src_l_counts);
        init_ref_counts(monomial(eq.r), m_src_r_counts);
        unsigned min_r = UINT_MAX;
        node* min_n = nullptr;
        for (auto n : monomial(eq.l))
            if (n->eqs.size() < min_r)
                min_n = n, min_r = n->eqs.size();
        // found node that occurs in fewest eqs
        VERIFY(min_n);
        return min_n->eqs;
    }

    void ac_plugin::init_ref_counts(monomial_t const& monomial, ref_counts& counts) const {
        init_ref_counts(monomial.m_nodes, counts);
    }

    void ac_plugin::init_ref_counts(ptr_vector<node> const& monomial, ref_counts& counts) const {
        counts.reset();
        for (auto n : monomial)
            counts.inc(n->id(), 1);
    }

    bool ac_plugin::is_correct_ref_count(monomial_t const& m, ref_counts const& counts) const {
        return is_correct_ref_count(m.m_nodes, counts);
    }

    bool ac_plugin::is_correct_ref_count(ptr_vector<node> const& m, ref_counts const& counts) const {
        ref_counts check;
        init_ref_counts(m, check);
        return
            all_of(counts, [&](unsigned i) { return check[i] == counts[i]; }) &&
            all_of(check, [&](unsigned i) { return check[i] == counts[i]; });
    }

    void ac_plugin::backward_simplify(unsigned src_eq, unsigned dst_eq) {

        if (src_eq == dst_eq)
            return;

        // check that left src.l is a subset of dst.r
        // dst = A -> BC 
        // src = B -> D  
        // post(dst) := A -> CD
        auto& src = m_active[src_eq];  // src_r_counts, src_l_counts are initialized
        auto& dst = m_active[dst_eq];

        TRACE(plugin_verbose, tout << "forward simplify " << eq_pp_ll(*this, src) << " " << eq_pp_ll(*this, dst) << "\n");

        if (backward_subsumes(src_eq, dst_eq)) {
            TRACE(plugin_verbose, tout << "forward subsumed\n");
            set_status(dst_eq, eq_status::is_dead_eq);
            return;
        }

        if (!can_be_subset(monomial(src.l), monomial(dst.r)))
            return;

        m_dst_r_counts.reset();

        unsigned src_l_size = monomial(src.l).size();
        unsigned src_r_size = m_src_r.size();

        SASSERT(is_correct_ref_count(monomial(src.l), m_src_l_counts));
        // subtract src.l from dst.r if src.l is a subset of dst.r
        // dst_rhs := dst_rhs - src_lhs + src_rhs
        //         := src_rhs + (dst_rhs - src_lhs)
        //         := src_rhs + elements from dst_rhs that are in excess of src_lhs
        unsigned num_overlap = 0;
        for (auto n : monomial(dst.r)) {
            unsigned id = n->id();
            unsigned src_count = m_src_l_counts[id];
            unsigned dst_count = m_dst_r_counts[id];
            if (dst_count < src_count) {
                m_dst_r_counts.inc(id, 1);
                ++num_overlap;
            }
            else
                m_src_r.push_back(n);
        }
        // The dst.r has to be a superset of src.l, otherwise simplification does not apply
        if (num_overlap != src_l_size) {
            m_src_r.shrink(src_r_size);
            return;
        }
        auto j = justify_rewrite(src_eq, dst_eq);
        reduce(m_src_r, j);
        auto new_r = to_monomial(m_src_r);
        index_new_r(dst_eq, monomial(m_active[dst_eq].r), monomial(new_r));
        m_active[dst_eq].r = new_r;
        m_active[dst_eq].j = j;
        m_src_r.reset();
        m_src_r.append(monomial(src.r).m_nodes);
        TRACE(plugin_verbose, tout << "rewritten to " << m_pp_ll(*this, monomial(new_r)) << "\n");
        m_new_eqs.push_back(dst_eq);
    }

    bool ac_plugin::forward_simplify(unsigned dst_eq, unsigned src_eq) {
        if (src_eq == dst_eq)
            return false;

        auto& src = m_active[src_eq];
        auto& dst = m_active[dst_eq]; // pre-computed dst_r_counts, dst_l_counts 
        //
        // dst_ids, dst_count contain rhs of dst_eq
        //
        TRACE(plugin, tout << "forward simplify " << eq_pp_ll(*this, src) << " " << eq_pp_ll(*this, dst) << " can-be-subset: " << can_be_subset(monomial(src.l), monomial(dst.r)) << "\n");

        if (forward_subsumes(src_eq, dst_eq)) {
            set_status(dst_eq, eq_status::is_dead_eq);
            return true;
        }
        SASSERT(!are_equal(m_active[src_eq], m_active[dst_eq]));
        
        if (!is_equation_oriented(src))
            return false;
        // check that src.l is a subset of dst.r
        if (!can_be_subset(monomial(src.l), monomial(dst.r)))
            return false;
        if (!is_subset(m_dst_r_counts, m_src_l_counts, monomial(src.l)))
            return false;
        if (monomial(dst.r).size() == 0)
            return false;


        SASSERT(is_correct_ref_count(monomial(dst.r), m_dst_r_counts));

        ptr_vector<node> m(m_dst_r);
        init_ref_counts(monomial(src.l), m_src_l_counts);

        //verbose_stream() << "forward simplify " << eq_pp_ll(*this, src_eq) << " for " << eq_pp_ll(*this, dst_eq) << "\n";

        rewrite1(m_src_l_counts, monomial(src.r), m_dst_r_counts, m);
        auto j = justify_rewrite(src_eq, dst_eq);
        reduce(m, j);
        auto new_r = to_monomial(m);
        index_new_r(dst_eq, monomial(m_active[dst_eq].r), monomial(new_r));
        m_active[dst_eq].r = new_r;
        m_active[dst_eq].j = j;
        TRACE(plugin, tout << "rewritten to " << m_pp(*this, monomial(new_r)) << "\n");

        return true;
    }

    void ac_plugin::forward_subsume_new_eqs() {
        for (auto f_id : m_new_eqs) {
            if (is_forward_subsumed(f_id))
                set_status(f_id, eq_status::is_dead_eq);
            else if (is_reducing(f_id)) {
                set_status(f_id, eq_status::is_reducing_eq);
                backward_reduce(f_id);
            }
            // set passive or active?
        }
        m_new_eqs.reset();
    }

    bool ac_plugin::is_forward_subsumed(unsigned eq_id) {
        return any_of(forward_iterator(eq_id), [&](unsigned other_eq) { return forward_subsumes(other_eq, eq_id); });
    }

    // dst_eq is fixed, dst_l_count is pre-computed for monomial(dst.l)
    // dst_r_counts is pre-computed for monomial(dst.r).
    // is dst_eq subsumed by src_eq?
    bool ac_plugin::forward_subsumes(unsigned src_eq, unsigned dst_eq) {
        auto& src = m_active[src_eq];
        auto& dst = m_active[dst_eq];
        TRACE(plugin_verbose, tout << "forward subsumes " << eq_pp_ll(*this, src) << " " << eq_pp_ll(*this, dst) << "\n");
        SASSERT(is_correct_ref_count(monomial(dst.l), m_dst_l_counts));
        SASSERT(is_correct_ref_count(monomial(dst.r), m_dst_r_counts));
        if (!can_be_subset(monomial(src.l), monomial(dst.l))) {
            TRACE(plugin_verbose, tout << "not subset of dst.l\n");
            SASSERT(!are_equal(m_active[src_eq], m_active[dst_eq]));
            return false;
        }
        if (!can_be_subset(monomial(src.r), monomial(dst.r))) {
            TRACE(plugin_verbose, tout << "not subset of dst.r\n");
            SASSERT(!are_equal(m_active[src_eq], m_active[dst_eq]));
            return false;
        }
        unsigned size_diff = monomial(dst.l).size() - monomial(src.l).size();
        if (size_diff != monomial(dst.r).size() - monomial(src.r).size()) {
            TRACE(plugin_verbose, tout << "size diff does not match: " << size_diff << "\n");
            SASSERT(!are_equal(m_active[src_eq], m_active[dst_eq]));
            return false;
        }
        if (!is_subset(m_dst_l_counts, m_src_l_counts, monomial(src.l))) {
            TRACE(plugin_verbose, tout << "not subset of dst.l counts\n");
            SASSERT(!are_equal(m_active[src_eq], m_active[dst_eq]));
            return false;
        }
        if (!is_subset(m_dst_r_counts, m_src_r_counts, monomial(src.r))) {
            TRACE(plugin_verbose, tout << "not subset of dst.r counts\n");
            SASSERT(!are_equal(m_active[src_eq], m_active[dst_eq]));
            return false;
        }
        SASSERT(is_correct_ref_count(monomial(src.l), m_src_l_counts));
        SASSERT(is_correct_ref_count(monomial(src.r), m_src_r_counts));
        SASSERT(is_correct_ref_count(monomial(dst.l), m_dst_l_counts));
        SASSERT(is_correct_ref_count(monomial(dst.r), m_dst_r_counts));
        // add difference between dst.l and src.l to both src.l, src.r
        for (auto n : monomial(dst.l)) {
            unsigned id = n->id();
            SASSERT(m_dst_l_counts[id] >= m_src_l_counts[id]);
            unsigned diff = m_dst_l_counts[id] - m_src_l_counts[id];
            if (diff > 0) {
                m_src_l_counts.inc(id, diff);
                m_src_r_counts.inc(id, diff);
            }
        }
        // now dst.r and src.r should align and have the same elements.
        // since src.r is a subset of dst.r we iterate over dst.r
        if (!all_of(monomial(src.r), [&](node* n) {
            unsigned id = n->id();
            return m_src_r_counts[id] == m_dst_r_counts[id]; })) {
            TRACE(plugin_verbose, tout << "dst.r and src.r do not align\n");
            SASSERT(!are_equal(m_active[src_eq], m_active[dst_eq]));
            return false;
        }
        bool r = all_of(monomial(dst.r), [&](node* n) { unsigned id = n->id(); return m_src_r_counts[id] == m_dst_r_counts[id]; });
        SASSERT(r || !are_equal(m_active[src_eq], m_active[dst_eq]));
        return r;
    }

    // src_l_counts, src_r_counts are initialized for src.l, src.r
    bool ac_plugin::backward_subsumes(unsigned src_eq, unsigned dst_eq) {
        auto& src = m_active[src_eq];
        auto& dst = m_active[dst_eq];
        SASSERT(is_correct_ref_count(monomial(src.l), m_src_l_counts));
        SASSERT(is_correct_ref_count(monomial(src.r), m_src_r_counts));
        if (!can_be_subset(monomial(src.l), monomial(dst.l)))
            return false;
        if (!can_be_subset(monomial(src.r), monomial(dst.r)))
            return false;
        unsigned size_diff = monomial(dst.l).size() - monomial(src.l).size();
        if (size_diff != monomial(dst.r).size() - monomial(src.r).size())
            return false;
        if (!is_superset(m_src_l_counts, m_dst_l_counts, monomial(dst.l)))
            return false;
        if (!is_superset(m_src_r_counts, m_dst_r_counts, monomial(dst.r)))
            return false;
        SASSERT(is_correct_ref_count(monomial(dst.l), m_dst_l_counts));
        SASSERT(is_correct_ref_count(monomial(dst.r), m_dst_r_counts));
        for (auto n : monomial(src.l)) {
            unsigned id = n->id();
            SASSERT(m_src_l_counts[id] <= m_dst_l_counts[id]);
            unsigned diff = m_dst_l_counts[id] - m_src_l_counts[id];
            if (diff == 0)
                continue;
            m_dst_l_counts.dec(id, diff);
            if (m_dst_r_counts[id] < diff)
                return false;
            m_dst_r_counts.dec(id, diff);
        }
        return all_of(monomial(dst.r), [&](node* n) { unsigned id = n->id(); return m_src_r_counts[id] == m_dst_r_counts[id]; });
    }

    void ac_plugin::rewrite1(ref_counts const& src_l, monomial_t const& src_r, ref_counts& dst_counts, ptr_vector<node>& dst) {
        // pre-condition: is-subset is invoked so that src_l is initialized.
        // pre-condition: dst_count is also initialized.
        // remove from dst elements that are in src_l   
        // add elements from src_r  
        SASSERT(is_correct_ref_count(dst, dst_counts));
        SASSERT(&src_r.m_nodes != &dst);
        unsigned sz = dst.size(), j = 0;
        for (unsigned i = 0; i < sz; ++i) {
            auto* n = dst[i];
            unsigned id = n->id();
            unsigned dst_count = dst_counts[id];
            unsigned src_count = src_l[id];
            SASSERT(dst_count > 0);

            if (src_count == 0) {
                dst[j++] = n;
            }
            else if (src_count < dst_count) {
                dst[j++] = n;
                dst_counts.dec(id, 1);
            }
        }
        dst.shrink(j);
        dst.append(src_r.m_nodes);
    }

    // rewrite monomial to normal form.
    bool ac_plugin::reduce(ptr_vector<node>& m, justification& j) {
        bool change = false;
        do {
        init_loop:
            bloom b;
            init_ref_counts(m, m_m_counts);
            for (auto n : m) {
                if (n->is_zero) {
                    m[0] = n;
                    m.shrink(1);
                    change = true;
                    break;
                }
                for (auto eq : n->eqs) {
                    auto& src = m_active[eq];

                    if (!is_equation_oriented(src))
                        continue;
                    if (!can_be_subset(monomial(src.l), m, b))
                        continue;
                    if (!is_subset(m_m_counts, m_eq_counts, monomial(src.l)))
                        continue;

                    TRACE(plugin, display_equation_ll(tout << "reduce ", src) << "\n");
                    SASSERT(is_correct_ref_count(monomial(src.l), m_eq_counts));
                    for (auto n : m)
                        for (auto s : n->shared)
                            m_shared_todo.insert(s);
                    rewrite1(m_eq_counts, monomial(src.r), m_m_counts, m);
                    j = join(j, eq);
                    change = true;
                    goto init_loop;
                }
            }
        } while (false);
        return change;
    }

    // check that src is a subset of dst, where dst_counts are precomputed
    bool ac_plugin::is_subset(ref_counts const& dst_counts, ref_counts& src_counts, monomial_t const& src) {
        SASSERT(&dst_counts != &src_counts);
        init_ref_counts(src, src_counts);
        return all_of(src_counts, [&](unsigned idx) { return src_counts[idx] <= dst_counts[idx]; });
    }

    // check that dst is a superset of src, where src_counts are precomputed
    bool ac_plugin::is_superset(ref_counts const& src_counts, ref_counts& dst_counts, monomial_t const& dst) {
        SASSERT(&dst_counts != &src_counts);
        init_ref_counts(dst, dst_counts);
        return all_of(src_counts, [&](unsigned idx) { return src_counts[idx] <= dst_counts[idx]; });
    }

    void ac_plugin::index_new_r(unsigned eq, monomial_t const& old_r, monomial_t const& new_r) {
        for (auto n : old_r)
            n->n->mark2();
        for (auto n : new_r) {
            if (!n->n->is_marked2()) {
                n->eqs.push_back(eq);
                m_node_trail.push_back(n);
                n->n->mark2();
                push_undo(undo_kind::is_add_eq_index);
            }
        }
        for (auto n : old_r)
            n->n->unmark2();
        for (auto n : new_r)
            n->n->unmark2();
    }

    bool ac_plugin::superpose(unsigned src_eq, unsigned dst_eq) {
        if (src_eq == dst_eq)
            return false;
        auto& src = m_active[src_eq];
        auto& dst = m_active[dst_eq];

        unsigned max_left = std::max(monomial(src.l).size(), monomial(dst.l).size());
        unsigned min_right = std::max(monomial(src.r).size(), monomial(dst.r).size());


        // AB -> C, AD -> E => BE ~ CD
        // m_src_ids, m_src_counts contains information about src (call it AD -> E)
        m_dst_l_counts.reset();

        m_dst_r.reset();
        m_dst_r.append(monomial(dst.r).m_nodes);
        unsigned src_r_size = m_src_r.size();
        SASSERT(src_r_size == monomial(src.r).size());
        // dst_r contains C
        // src_r contains E

        // compute BE, initialize dst_ids, dst_counts
        bool overlap = false;
        for (auto n : monomial(dst.l)) {
            unsigned id = n->id();
            m_dst_l_counts.inc(id, 1);
            if (m_src_l_counts[id] < m_dst_l_counts[id])
                m_src_r.push_back(n);
            overlap |= m_src_l_counts[id] > 0;
        }

        if (!overlap) {
            m_src_r.shrink(src_r_size);
            return false;
        }

        // compute CD
        for (auto n : monomial(src.l)) {
            unsigned id = n->id();
            if (m_dst_l_counts[id] > 0)
                m_dst_l_counts.dec(id, 1);
            else
                m_dst_r.push_back(n);
        }

        if (are_equal(m_src_r, m_dst_r)) {
            m_src_r.shrink(src_r_size);
            return false;
        }

        justification j = justify_rewrite(src_eq, dst_eq);
        reduce(m_dst_r, j);
        reduce(m_src_r, j);
        deduplicate(m_src_r, m_dst_r);


        bool added_eq = false;
        auto src_r = src.r;
        unsigned max_left_new = std::max(m_src_r.size(), m_dst_r.size());
        unsigned min_right_new = std::min(m_src_r.size(), m_dst_r.size());
        if (max_left_new <= max_left && min_right_new <= min_right)
            added_eq = init_equation(eq(to_monomial(m_src_r), to_monomial(m_dst_r), j), false);

        CTRACE(plugin, added_eq,
            tout << "superpose: " << m_name << " " << eq_pp_ll(*this, src) << " " << eq_pp_ll(*this, dst) << " --> ";
        tout << m_pp_ll(*this, m_src_r) << "== " << m_pp_ll(*this, m_dst_r) << "\n";);

        m_src_r.reset();
        m_src_r.append(monomial(src_r).m_nodes);
        return added_eq;
    }

    bool ac_plugin::is_reducing(eq const& e) const {
        auto const& l = monomial(e.l);
        auto const& r = monomial(e.r);
        return l.size() == 1 && r.size() <= 1;
    }

    void ac_plugin::backward_reduce(unsigned eq_id) {
        auto const& eq = m_active[eq_id];
        if (!is_reducing(eq))
            return;
        for (auto other_eq : superpose_iterator(eq_id)) {
            SASSERT(is_active(other_eq));
            backward_reduce(eq, other_eq);
        }
        for (auto p : m_passive) {
            bool change = false;
            if (backward_reduce_monomial(eq, p, monomial(p.l)))
                change = true;
            if (backward_reduce_monomial(eq, p, monomial(p.r)))
                change = true;
            (void)change;
            CTRACE(plugin, change,
                verbose_stream() << "backward reduce " << eq_pp_ll(*this, eq) << " " << eq_pp_ll(*this, p) << "\n");
        }           
    }

    void ac_plugin::backward_reduce(eq const& eq, unsigned other_eq_id) {
        auto& other_eq = m_active[other_eq_id];
        TRACE(plugin_verbose,
            tout << "backward reduce " << eq_pp_ll(*this, eq) << " " << eq_pp_ll(*this, other_eq) << "\n");
        bool change = false;
        if (backward_reduce_monomial(eq, other_eq, monomial(other_eq.l)))
            change = true;
        if (backward_reduce_monomial(eq, other_eq, monomial(other_eq.r)))
            change = true;
        CTRACE(plugin, change,
            tout << "backward reduce " << eq_pp_ll(*this, eq) << " " << eq_pp_ll(*this, other_eq) << "\n");
        if (change) {
            set_status(other_eq_id, eq_status::is_to_simplify_eq);
        }
    }

    bool ac_plugin::backward_reduce_monomial(eq const& src, eq& dst, monomial_t& m) {
        auto const& r = monomial(src.r);
        unsigned j = 0;
        bool change = false;
        for (auto n : m) {
            unsigned id = n->id();
            SASSERT(m_src_l_counts[id] <= 1);
            if (m_src_l_counts[id] == 0) {
                m.m_nodes[j++] = n;
                continue;
            }
            change = true;
            for (auto s : n->shared)
                m_shared_todo.insert(s);
            if (r.size() == 0)
                // if r is empty, we can remove n from l
                continue;
            SASSERT(r.size() == 1);
            if (r[0]->is_zero) {
                m.m_nodes[0] = r[0];
                j = 1;
                break;
            }
            m.m_nodes[j++] = r[0];
        }
        m.m_nodes.shrink(j);
        if (change) {
            m.m_bloom.m_tick = 0;
            dst.j = join(dst.j, src);
        }
        return change;
    }

    bool ac_plugin::are_equal(monomial_t& a, monomial_t& b) {
        return filter(a) == filter(b) && are_equal(a.m_nodes, b.m_nodes);
    }

    bool ac_plugin::are_equal(ptr_vector<node> const& a, ptr_vector<node> const& b) {
        if (a.size() != b.size())
            return false;
        m_eq_counts.reset();
        for (auto n : a)
            m_eq_counts.inc(n->id(), 1);
        for (auto n : b) {
            unsigned id = n->id();
            if (m_eq_counts[id] == 0)
                return false;
            m_eq_counts.dec(id, 1);
        }
        return true;
    }

    bool ac_plugin::well_formed(eq const& e) const {
        if (e.l == e.r)
            return false; // trivial equation
        for (auto n : monomial(e.l)) {
            if (n->is_zero && monomial(e.l).size() > 1)
                return false; // zero is not allowed in equations
        }
        for (auto n : monomial(e.r)) {
            if (n->is_zero && monomial(e.r).size() > 1)
                return false; // zero is not allowed in equations
        }
        return true;
    }


    void ac_plugin::deduplicate(monomial_t& a, monomial_t& b) {
        unsigned sza = a.size(), szb = b.size();
        deduplicate(a.m_nodes, b.m_nodes);
        if (sza != a.size())
            a.m_bloom.m_tick = 0;
        if (szb != b.size())
            b.m_bloom.m_tick = 0;
    }

    void ac_plugin::deduplicate(ptr_vector<node>& a, ptr_vector<node>& b) {
        for (auto n : a) {
            if (n->is_zero) {
                a[0] = n;
                a.shrink(1);
                break;
            }
        }
        for (auto n : b) {
            if (n->is_zero) {
                b[0] = n;
                b.shrink(1);
                break;
            }
        }

        if (!m_is_injective)
            return;
        m_eq_counts.reset();
        for (auto n : a)
            m_eq_counts.inc(n->id(), 1);
        bool has_dup = any_of(b, [&](node* n) { return m_eq_counts[n->id()] > 0; });
        if (!has_dup)
            return;
        std::sort(a.begin(), a.end(), [&](node* x, node* y) { return x->id() < y->id(); });
        std::sort(b.begin(), b.end(), [&](node* x, node* y) { return x->id() < y->id(); });
        unsigned i = 0, j = 0, in = 0, jn = 0;
        for (; i < a.size() && j < b.size(); ) {
            if (a[i]->id() == b[j]->id()) {
                ++i;
                ++j;
            }
            else if (a[i]->id() < b[j]->id()) {
                a[in++] = a[i++];
            }
            else {
                b[jn++] = b[j++];
            }
        }
        for (; i < a.size(); ++i)
            a[in++] = a[i];
        for (; j < b.size(); ++j)
            b[jn++] = b[j];
        a.shrink(in);
        b.shrink(jn);
    }

    //
    // simple version based on propagating all shared
    // todo: version touching only newly processed shared, and maintaining incremental data-structures.
    //       - hash-tables for shared monomials similar to the ones used for euf_table.
    //         the tables have to be updated (and re-sorted) whenever a child changes root.
    // 

    void ac_plugin::propagate_shared() {
        TRACE(plugin_verbose, tout << "num shared todo " << m_shared_todo.size() << "\n");
        if (m_shared_todo.empty())
            return;

        while (!m_shared_todo.empty()) {
            auto idx = *m_shared_todo.begin();
            m_shared_todo.remove(idx);        
            TRACE(plugin, tout << "index " << idx << " shared size " << m_shared.size() << "\n");
            if (idx < m_shared.size())
                simplify_shared(idx, m_shared[idx]);
        }
        m_monomial_table.reset();
        for (auto const& s1 : m_shared) {
            shared s2;
            TRACE(plugin_verbose, tout << "shared " << s1.m << ": " << m_pp_ll(*this, monomial(s1.m)) << "\n");
            if (!m_monomial_table.find(s1.m, s2))
                m_monomial_table.insert(s1.m, s1);
            else if (s2.n->get_root() != s1.n->get_root()) {
                TRACE(plugin, tout << "merge shared " << g.bpp(s1.n->get_root()) << " and " << g.bpp(s2.n->get_root()) << "\n");
                push_merge(s1.n, s2.n, justification::dependent(m_dep_manager.mk_join(m_dep_manager.mk_leaf(s1.j), m_dep_manager.mk_leaf(s2.j))));
            }
        }
    }

    void ac_plugin::simplify_shared(unsigned idx, shared s) {
        auto j = s.j;
        auto old_m = s.m;
        auto old_n = monomial(old_m).m_src;
        ptr_vector<node> m1(monomial(old_m).m_nodes);
        TRACE(plugin, tout << "simplify shared: " << g.bpp(old_n) << ": " << m_pp_ll(*this, monomial(old_m)) << "\n");
        if (!reduce(m1, j))
            return;

        enode* new_n = nullptr;
        new_n = m1.empty() ? get_unit(s.n) : from_monomial(m1);
        auto new_m = to_monomial(new_n, m1);
        // update shared occurrences for members of the new monomial that are not already in the old monomial.
        for (auto n : monomial(old_m))
            n->n->mark2();
        for (auto n : m1) {
            if (!n->n->is_marked2()) {
                n->shared.push_back(idx);
                m_shared_todo.insert(idx);
                m_node_trail.push_back(n);
                push_undo(undo_kind::is_add_shared_index);
            }
        }
        for (auto n : monomial(old_m))
            n->n->unmark2();
        m_update_shared_trail.push_back({ idx, s });
        push_undo(undo_kind::is_update_shared);
        m_shared[idx].m = new_m;
        m_shared[idx].j = j;
        TRACE(plugin_verbose, tout << "shared simplified to " << m_pp_ll(*this, monomial(new_m)) << "\n");
        push_merge(old_n, new_n, j);
    }

    justification ac_plugin::justify_rewrite(unsigned eq1, unsigned eq2) {
        auto* j = m_dep_manager.mk_join(justify_equation(eq1), justify_equation(eq2));
        return justification::dependent(j);
    }

    justification::dependency* ac_plugin::justify_equation(unsigned eq) {
        return m_dep_manager.mk_leaf(m_active[eq].j);
    }

    justification ac_plugin::join(justification j, unsigned eq) {
        return justification::dependent(m_dep_manager.mk_join(m_dep_manager.mk_leaf(j), justify_equation(eq)));
    }

    justification ac_plugin::join(justification j, eq const& eq) {
        return justification::dependent(m_dep_manager.mk_join(m_dep_manager.mk_leaf(j), m_dep_manager.mk_leaf(eq.j)));
    }

}