An object ?id is the identity element for binary operator ?o over domain ?d iff for every instance ?x of ?d, applying ?o to ?x and ?id results in ?x.
(<=> (Identity-Element-For ?Id ?Op ?Domain)
(And (Instance-Of ?Id ?Domain)
(Binary-Operator-On ?Op ?Domain)
(Forall (?X)
(=> (Instance-Of ?X ?Domain)
(= (Value ?Op ?Id ?X) ?X)))))