(* Product Category *)
(** The product of two categories has as objects pairs of objects (first component from the first category and the second component from the second category) and as arrows pairs of arrows. *)
Local Open Scope morphism_scope.
Local Obligation Tactic := idtac.
Program Definition Prod_Cat (C C' : Category) : Category :=
{|
Obj := C * C';
Hom :=
fun a b =>
(((fst a) –≻ (fst b)) * ((snd a) –≻ (snd b)))%type;
compose :=
fun a b c f g =>
(((fst g) ∘ (fst f)), ((snd g) ∘ (snd f)))%morphism;
id := fun c => (id, id)
|}.
Next Obligation.
cbn.
intros.
rewrite (assoc (fst _)).
rewrite assoc.
trivial.
Defined.
Next Obligation.
cbn.
intros C C' a b c d f g h.
rewrite (assoc_sym (fst _)).
rewrite assoc_sym.
trivial.
Defined.
Next Obligation.
cbn.
intros.
rewrite (id_unit_left (fst _)).
rewrite id_unit_left.
trivial.
Defined.
Next Obligation.
cbn.
intros.
rewrite (id_unit_right (fst _)).
rewrite id_unit_right.
trivial.
Defined.
Notation "C × D" := (Prod_Cat C D) : category_scope.
Local Obligation Tactic := basic_simpl; auto.

Theorem Prod_compose_id
(C D : Category)
(a b c : C) (d : D)
(f : a –≻ b) (g : b –≻ c)
: (g ∘ f, id d)%morphism =
@compose (_ × _) (_, _) (_, _) (_, _) (f, id d) (g, id d).