proposition: Lemma msetDLVR x A B : x \in A `+` B -> (x \in A) + (x \in B).
proof: Proof. by rewrite inE; case: (x \in A); [left|right]. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap