proposition: Lemma imset_union_set_eq: forall (U T : Type) (s1 s2 : set U) (f : U -> T), f @: (s1 :|: s2) <--> f @: s1 :|: f @: s2.
proof: Proof. intros U T s1 s2 f. firstorder. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)