SWI-Prolog -- ?= / 2

Documentation
- Reference manual
  - Built-in Predicates
    - Comparison and Unification of Terms
      - Special unification and comparison predicates
        
        unify_with_occurs_check/2
        
        =@=/2
        
        \=@=/2
        
        subsumes_term/2
        
        term_subsumer/3
        
        unifiable/3
        
        ?=/2
- Packages

Availability:built-in

?=(@Term1, @Term2)

Succeeds if the syntactic equality of Term1 and Term2 can be decided safely, i.e. if the result of Term1 == Term2 will not change due to further instantiation of either term. It behaves as if defined by ?=(X,Y) :- \+ unifiable(X,Y,[_|_]).