proposition: Lemma ltr_nwDr y x z : x < 0 -> y <= z -> y + x < z.
proof: Proof. by move=> *; rewrite addrC ltr_nwDl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)