Section Commutative_rings.
Variable R1 : CRING.
exact (cring_com_prf R1).
Qed.
Hint Immediate CRING_com: algebra.

Lemma CRING_mult4 :
forall a b c d : R1,
Equal (ring_mult (ring_mult a b) (ring_mult c d))
(ring_mult (ring_mult a c) (ring_mult b d)).