Section PermSeq.
Variable T : eqType.
Implicit Type s : seq T.
Definition perm_eq s1 s2 :=
all [pred x | count_mem x s1 == count_mem x s2] (s1 ++ s2).

Lemma perm_undup s1 s2 : s1 =i s2 -> perm_eq (undup s1) (undup s2).