proposition: Lemma mem_allpairs_consr f s t y0 : [seq f x y | x <- s, y <- y0 x :: t x] =i [seq f x (y0 x) | x <- s] ++ [seq f x y | x <- s, y <- t x].
proof: Proof. exact/perm_mem/permPl/perm_allpairs_consr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)