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