proposition: Lemma bigcup_set_I_l {A B} (s1 s2 : set A) (s3 : set B) (f : A -> set B) : \bigcup_(x in s1) (f x) \subset s3 -> \bigcup_(x in (s1 :&: s2)) (f x) \subset s3.
proof: Proof. firstorder. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)