proposition: Lemma fset1Ur x a B : x \in B -> x \in a |` B.
proof: Proof. by move=> Bx; rewrite !inE predU1r. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)