proposition: Lemma csum_nz a b: a +c b = \0c -> (a = \0c /\ b = \0c).
proof: Proof. move/csum_zero_unit_bis; rewrite/allf; aw => h. by move:(h _ inc_C0_C2)(h _ inc_C1_C2); aw. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)