1	a =@= A	false
2	A =@= B	true
3	x(A,A) =@= x(B,C)	false
4	x(A,A) =@= x(B,B)	true
5	x(A,A) =@= x(A,B)	false
6	x(A,B) =@= x(C,D)	true
7	x(A,B) =@= x(B,A)	true
8	x(A,B) =@= x(C,A)	true

A term is always a variant of a copy of itself. Term copying takes place in, e.g., copy_term/2, findall/3 or proving a clause added with asserta/1. In the pure Prolog world (i.e., without attributed variables), =@=/2 behaves as if defined below. With attributed variables, variant of the attributes is tested rather than trying to satisfy the constraints.

A =@= B :-
        copy_term(A, Ac),
        copy_term(B, Bc),
        numbervars(Ac, 0, N),
        numbervars(Bc, 0, N),
        Ac == Bc.

The SWI-Prolog implementation is cycle-safe and can deal with variables that are shared between the left and right argument. Its performance is comparable to ==/2, both on success and (early) failure. ^{66The current implementation is contributed by Kuniaki Mukai.}

This predicate is known by the name variant/2 in some other Prolog systems. Be aware of possible differences in semantics if the arguments contain attributed variables or share variables.^{67In many systems variant is implemented using two calls to subsumes_term/2.}