asFail
Casts the current Substitution to Fail, if possible, or returns null otherwise
Return
the current Substitution, casted to Fail, or null, if the current term is not an instance of Fail
Casts the current Substitution to Fail, if possible, or returns null otherwise
the current Substitution, casted to Fail, or null, if the current term is not an instance of Fail