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 : forall (n m : nat) (x y : list nat), nocc n x = nocc n y -> nocc n (Cons nat m x) = nocc n (Cons nat m y).