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

Theorem sorted_ins_Cons : forall (x : list nat) (n m : nat), inf_egal n m = false -> sorted (Cons nat m x) -> sorted (Cons nat m (ins n x)).