hnf_compound_head.chr

:- module(hnf_compound_head, [pair/2, tri/2, deep/2, mismatch/2, type(pat/0)]).
:- chr_constraint pair/2, tri/2, deep/2, mismatch/2.
:- chr_type pat ---> p(any, any) ; q(any) ; h(any) ; f(any) ; g(any).

% Pair head: extract both inner vars, return them in the result.
pair(p(X, Y), R) <=> R = result(X, Y).
pair(_, R)       <=> R = no_match.

% Triple-nested compound: inner var must thread through three levels.
tri(f(g(h(X))), R) <=> R = X.
tri(_, R)          <=> R = no_match.

% Mixed-shape head: the second arg is also a compound.
deep(p(X, q(Y)), R) <=> R = pair_q(X, Y).
deep(_, R)          <=> R = no_match.

% Pattern that should NOT match: probe falls through to the third rule.
mismatch(f(_), R) <=> R = matched_f.
mismatch(g(_), R) <=> R = matched_g.
mismatch(_, R)    <=> R = none.