Hint Resolve Permutation_map.

(* Specifications: *)
Lemma map_cons T U (f: T -> U) (h: T) (l: list T): map f (h :: l) = f h :: map f l.
Lemma In_concat X (l: list (list X)) (s: list X) (x: X): In x s -> In s l -> In x (concat l).
Proof with auto.