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