proposition: Lemma led_add a b c d : a <= c -> b <= d -> a + b <= c + d.
proof: Proof. move=> Hac Hbd; exact/(le_trans (led_add2r _ Hac)) /led_add2l. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype choice order) mathcomp(ssrnat bigop ssralg)