proposition: Lemma add_le3 a b: a = a + b -> b = zero.
proof: Proof. move /eqP;rewrite -{1} (T3addn0 a) T3eq_add2l => /eqP -> //. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)