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 eq_count_undup a s1 s2 :
{in a, s1 =i s2} -> count a (undup s1) = count a (undup s2).