rattac.
arithNormalize.
f_equal.
f_equal.
rewrite mult_comm.
rewrite (mult_comm _ x1).
repeat rewrite mult_assoc.
f_equal.
rewrite mult_comm.
rewrite e.
apply mult_comm.
do 3 arithSimplify.
Qed.

Theorem ratAdd_eqRat_compat : forall r1 r2 r3 r4,
eqRat r1 r2 ->
eqRat r3 r4 ->
r1 + r3 == r2 + r4.