proposition: Lemma prelift_cat s1 s2 : prelift (s1 ++ s2) = prelift s1 + prelift s2.
proof: Proof. exact: big_cat. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)