proposition: Lemma eq_proper A B : A =i B -> proper (mem A) =1 proper (mem B).
proof: Proof. move=> eAB [C]; congr (_ && _); first exact: (eq_subset eAB). by rewrite (eq_subset_r eAB). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path div)