proposition: Lemma can2_semi_additive f' : cancel f f' -> cancel f' f -> semi_additive f'.
proof: Proof. move=> fK f'K. by split=> [|x y]; apply: (canLR fK); rewrite ?raddf0// raddfD !f'K. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)