proposition: Lemma msubsetUl A B : A `<=` A `|` B.
proof: Proof. by apply/msubsetP=> a; rewrite !msetE leq_maxl. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap