Fixpoint tri_ins (l : list nat) : list nat :=
  match l with
  | Nil => Nil nat
  | Cons n x => ins n (tri_ins x)
  end.

Theorem nocc_Cons_Cons : forall (n m p : nat) (x : list nat), nocc n (Cons nat m (Cons nat p x)) = nocc n (Cons nat p (Cons nat m x)).