proposition: Lemma imset_bigcup T U V (f : U -> V) A (F : T -> set U) : (f @: \bigcup_(x in A) (F x)) <--> \bigcup_(x in A) f @: F x.
proof: Proof. by rewrite /imset bigcup_flatten. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)