proposition: Lemma fproperD1 A x : x \in A -> A `\ x `<` A.
proof: Proof. move=> Ax; rewrite fproperE fsubsetDl; apply/fsubsetPn; exists x=> //. by rewrite in_fsetD1 Ax eqxx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)