proposition: Lemma allpairs_mapr f (g : forall x, T' x -> T x) s t : [seq f x y | x <- s, y <- map (g x) (t x)] = [seq f x (g x y) | x <- s, y <- t x].
proof: Proof. by under eq_map do rewrite -map_comp. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)