proposition: Lemma fsetULVR 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: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)