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_size s1 s2 : perm_eq s1 s2 -> size s1 = size s2.