intros.elim x.
simplify.reflexivity.
simplify.rewrite > H. rewrite > assoc_plus.rewrite > assoc_plus.
-apply eq_f.rewrite < assoc_plus. rewrite < sym_plus ? z.
+apply eq_f.rewrite < assoc_plus. rewrite < (sym_plus ? z).
rewrite > assoc_plus.reflexivity.
qed.
simplify.rewrite < sym_times.
rewrite > distr_times_plus.
rewrite < sym_times.
-rewrite < sym_times (times n y) z.
+rewrite < (sym_times (times n y) z).
rewrite < H.apply refl_eq.
qed.