Struct Forest 

pub struct Forest {
    pub roots: Vec<usize>,
    pub children: Vec<Vec<usize>>,
    pub unreachable: Vec<usize>,
}

Parent/child forest over a set of items, classified by membership in that set.

The input is assumed to already be in canonical backlog order (BacklogItem::backlog_cmp); this type preserves that order so roots and siblings stay rank-ordered.

Fields

roots: Vec<usize>

Root item indices (no parent, parent outside the set, or self-parent), in input order.

children: Vec<Vec<usize>>

Direct children per item index (by input order); empty slice when none.

unreachable: Vec<usize>

Indices unreachable from any root (parent-link cycles), in input order.