-- wh >len q 0{...} — prefix-binop condition with a known-arity call on the
-- left operand. Six-rerun standing P1: pre-fix this errored on first
-- attempt across every persona, forcing bind-first or paren workarounds.
-- Post-fix, the natural shape parses directly because parse_prefix_binop
-- expands known-arity idents into calls (mirroring the prefix-`??` fix
-- in #310).
-- drain-tail: shrink a list until empty, count the iterations.
drain-tail xs:L n>n;c=0;wh >len xs 0{xs=tl xs;c=+c 1};c
-- bound-by-other: shrink until length matches a reference list's length.
bound-by-other xs:L n ys:L n>n;wh >len xs len ys{xs=tl xs};len xs
-- guard form: `>len q 1{...}` — the same prefix-call shape in a guard
-- header (not just `wh`).
guard-len q:L n>n;>len q 1{ret 42};0
-- prefix-ternary form: `?>len q 0 a b` — condition is a prefix-call.
ternary-len q:L n>n;?>len q 0 100 0
-- run: drain-tail [1,2,3]
-- out: 3
-- run: drain-tail []
-- out: 0
-- run: bound-by-other [1,2,3,4,5] [1,2]
-- out: 2
-- run: bound-by-other [1] [1,2,3]
-- out: 1
-- run: guard-len [1,2]
-- out: 42
-- run: guard-len []
-- out: 0
-- run: ternary-len [1,2,3]
-- out: 100
-- run: ternary-len []
-- out: 0