proposition: Lemma setI_set_incl : forall (A : Type) (s1 s2 s3 : set A), s1 \subset s2 -> s1 \subset s3 -> s1 \subset s2 :&: s3.
proof: Proof. firstorder. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)