proposition: Lemma subset_singl : forall {T} (x y : T), [set x] \subset [set y] <-> y = x.
proof: Proof. intros. split; intros H; subst; auto. - apply H; reflexivity. - apply subset_refl. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)