Definition addToPartitioning (c: comparison) (l: list T) (h: T) (H: {p: Partitioning | Permutation (p Eq ++ p Lt ++ p Gt) l}): {p: Partitioning | Permutation (p Eq ++ p Lt ++ p Gt) (h :: l)} :=
    exist (fun p => Permutation (p Eq ++ p Lt ++ p Gt) (h :: l))
      (fun c' => if cmp_cmp c c' then h :: proj1_sig H c' else proj1_sig H c')
      (partition_oblig c h H).Definition emp: {p: Partitioning | Permutation (p Eq ++ p Lt ++ p Gt) nil} :=
    exist (fun p => Permutation (p Eq ++ p Lt ++ p Gt) nil) (fun _ => nil) (perm_nil T).Fixpoint repeat T (n: nat) (x: T): list T :=
  match n with
  | 0 => nil
  | S n' => x :: repeat n' x
  end.

Lemma length_0_nil A (l: list A): length l = 0%nat <-> l = nil.