hnf_wildcard_in_head.chr

:- module(hnf_wildcard_in_head, [peek/3, two/3, mid/4]).
:- chr_constraint peek/3, two/3, mid/4.

% Wildcards in head positions; the named X is what gets returned.
peek(_, X, R)    <=> R = X.

% Multiple wildcards at different positions.
two(_, _, R)     <=> R = matched.

% Wildcard between two named arguments — the named ones must still bind.
mid(A, _, B, R)  <=> R = pair(A, B).