proposition: Lemma fsubsetD1P A B x : reflect (A `<=` B /\ x \notin A) (A `<=` B `\ x).
proof: Proof. by rewrite fsubsetD1; exact: andP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)