Section suml.
Fixpoint suml (l : list nat) : nat :=
match l with
| nil => 0
| n :: l' => n + suml l'
end.

Lemma Permutation_partition_list_nat b n l :
(forall x, In x l -> b <= x < b + n) ->
Permutation l (flat_map (fun n => filter (beq_nat n) l) (seq b n)).