proposition: Lemma predeg_cat D1 D2 : predeg (D1 ++ D2) = predeg D1 + predeg D2.
proof: Proof. by rewrite !predegE prelift_cat. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)