Section Permutations.
Variable T : eqType.
Implicit Types (x : T) (s t : seq T) (bs : seq (T * nat)) (acc : seq (seq T)).
Fixpoint incr_tally bs x :=
if bs isn't b :: bs then [:: (x, 1)] else
if x == b.1 then (x, b.2.+1) :: bs else b :: incr_tally bs x.
Definition tally s := foldl incr_tally [::] s.
Definition wf_tally :=
[qualify a bs : seq (T * nat) | uniq (unzip1 bs) && (0 \notin unzip2 bs)].
Definition tally_seq bs := flatten [seq nseq b.2 b.1 | b <- bs].
Local Notation tseq := tally_seq.

Lemma perm_tally s1 s2 : perm_eq s1 s2 -> perm_eq (tally s1) (tally s2).