proposition: Lemma in_mset1D a' A a : (a \in a' +` A) = (a == a') || (a \in A).
proof: Proof. by rewrite in_msetDU in_mset1U. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap