proposition: Lemma in_mset1U a' A a : (a \in a' |` A) = (a == a') || (a \in A).
proof: Proof. by rewrite in_msetU in_mset msetE; case: (_ == _). Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap