Definition jj : forall d : nat, mon d :=
  (fix jj (d : nat) : mon d :=
     match d as n return (mon n) with
     | O => n_0
     | S n => c_n n 0 (jj n)
     end).

Theorem jjProp1 : forall (d : nat) (m : mon d), d = 0 -> m = jj d.