underscore_var.chr

:- module(underscore_var, [head_use/2, list_tail/2, trivial/1]).
:- chr_constraint head_use/2, list_tail/2, trivial/1.

% _X in head position binds normally and is referenced in the body.
head_use(_X, R) <=> R = _X.

% _Tail in a list pattern is just an ordinary variable name.
list_tail([_H | _Tail], R) <=> R = _Tail.

% Repro from BUGS.md: parser must accept p(_X) <=> true.
trivial(_X) <=> true.