proposition: Lemma bigcup_nil_subset {A B} (f : A -> set B) s : \bigcup_(x in []) (f x) \subset s.
proof: Proof. intros x [y [H1 H2]]. inv H1. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)