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