Section Monomials.
Inductive mon : nat -> Set :=
| n_0 : mon 0
| c_n : forall d : nat, nat -> mon d -> mon (S d).
Definition pmon1 : forall d : nat, mon d -> nat.
(* Realizer [d: nat][m:(mon d)](Cases  m of n_0 => O | (c_n _ n _) => n end). *)
intros d m; case m.
exact 0.
intros d' n p; exact n.
Defined.
Definition pmon2 : forall d : nat, mon d -> mon (pred d).
(* Realizer [d:nat] [m:mon](<mon>Case m of n_0 [a:nat] [b:nat] [m':mon]m' end). *)
intros d m; case m.
exact n_0.
intros d' n p; exact p.
Defined.
Definition recomp : forall d : nat, d <> 0 -> mon d -> mon d.
intros d; case d.
intros H'; apply False_rec; case H'; auto.
intros d' H m; exact (c_n d' (pmon1 (S d') m) (pmon2 (S d') m)).
(*Realizer [d:nat] [m:mon] (<mon>Case d of (False_rec mon) [d':nat](c_n d' (pmon1 (S d') m) (pmon2 (S d') m)) end). Program_all. *)
Defined.
intros d h m.
generalize h; clear h.
elim m.
intros h; elim h; auto.
intros; simpl in |- *; unfold recomp in |- *; auto.
Qed.
intros d m; cut (S d <> 0); auto.
intros H; cut (recomp (S d) H m = m); auto.
apply recomp_ok; auto.
Qed.
(*Generator *)
Definition gen_mon : forall d : nat, nat -> mon d.
intros d; elim d.
intros n; exact n_0.
intros n H' n'; case n'.
exact (c_n n 1 (H' n)).
intros n''; exact (c_n n 0 (H' n'')).
Defined.
(* Multiplication of monomials. *)
Definition mult_mon : forall d : nat, mon d -> mon d -> mon d.
intros d; elim d.
intros H' H'0; exact n_0.
intros n Rec S1 S2.
exact
(c_n n (pmon1 (S n) S1 + pmon1 (S n) S2)
(Rec (pmon2 (S n) S1) (pmon2 (S n) S2))).
Defined.

Theorem mult_mon_com :
forall (d : nat) (a b : mon d), mult_mon d a b = mult_mon d b a.