True when Term1 and Term2 are terms that have the 
same functor (Name/Arity). The arguments must be 
sufficiently instantiated, which means either Term1 or Term2 
must be bound or both Name and Arity must be 
bound.
If Arity is 0, Term1 and Term2 are 
unified with Name for compatibility.
- Compatibility
- SICStus