proposition: Lemma setU_set0_l {A} (s1 s2 s3 : set A) : s1 <--> set0 -> s2 <--> s3 -> (s1 :|: s2) <--> s3.
proof: Proof. firstorder. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)