proposition: Lemma mset1UP x a B : reflect (x = a \/ x \in B) (x \in a |` B).
proof: Proof. by rewrite !inE; exact: predU1P. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap