?X/0¶

?Y is iso

Succeeds if terms X and Y are strictly identical. The difference between this predicate and = _50 "/2" is that, if one of the arguments is a free variable, it only succeeds when they have already been unified.

 ?- X == Y.

fails, but,

 ?- X = Y, X == Y.

succeeds.

 ?- X == 2.

fails, but,

 ?- X = 2, X == 2.

succeeds.