proposition: Lemma fsetU1l x A b : x \in A -> x \in A `|` [fset b].
proof: Proof. by move=> Ax; rewrite !inE Ax. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)