induction l...
simpl.
unfold Basics.compose.
rewrite length_concat.
rewrite map_map.
rewrite (@add_same (S (length l)) (map (fun x => length (insert_everywhere a x)) (perms l))).
rewrite map_length.
rewrite IHl.
rewrite Mult.mult_comm...
intros.
destruct (in_map_iff (fun x => length (insert_everywhere a x)) (perms l) x).
clear H1.
apply H0 in H. clear H0.
destruct H. destruct H.
subst.
rewrite length_insert_everywhere.
f_equal.
apply Permutation_length.
apply perms_are_perms...
Qed.
Definition alt_perms l: list (list T) :=
match l with
| nil => nil :: nil
| _ => concat (map (fun p => (map (cons (fst p)) (perms (snd p)))) (splits l))
end.

Lemma splits_permuted (l l': list T): Permutation l l' ->
Permuted (fun x y => fst x = fst y /\ Permutation (snd x) (snd y)) (splits l) (splits l').