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 permP s1 s2 : reflect (count^~ s1 =1 count^~ s2) (perm_eq s1 s2).