proposition: Lemma fsetD1P x A b : reflect (x != b /\ x \in A) (x \in A `\ b).
proof: Proof. by rewrite !inE; exact: andP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)