Quad testing for equal domains fails

### Description I'm trying to use quad to validate that two predicates share the same domain on a given argument position. Given: ```prolog foo(a). foo(b). foo(c). bar(a, 'ex:a'). bar(b, 'ex:b'). bar(c, 'ex:c'). ``` I want to check that the first argument of `bar/2` has the same domain as the first argument of `foo/1`: ```prolog ?- bar(X, _). foo(X). ``` ### Expected behavior The check should succeed, since every value in the first argument of `bar/2` (`a`, `b`, `c`) is also in the domain of `foo/1`. ### Actual behavior 1. I get a **singleton variable warning** for `X`. 2. The query ```prolog ?- check_modules_quads('my module', _). ``` returns `false`.