Local Hint Resolve Permutation_middle2 : core.

(* Specifications: *)
Lemma Permutation_elt : forall l1 l2 l1' l2' (a:A),
Permutation (l1 ++ l2) (l1' ++ l2') ->
Permutation (l1 ++ a :: l2) (l1' ++ a :: l2').
Theorem Permutation_rev : forall (l : list A), Permutation l (rev l).
Global Instance Permutation_rev' :
Proper (@Permutation A ==> @Permutation A) (@rev A).
Theorem Permutation_length : forall (l l' : list A),
Permutation l l' -> length l = length l'.