proposition: Lemma size_XaddC b : size ('X + b%:P) = 2.
proof: Proof. by rewrite -[b]opprK rmorphN size_XsubC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)