simple induction n; simpl in |- *; auto with *.
apply Trans with (group_inverse G r); auto with *.
intros n0 H'; try assumption.
apply
Trans
with (sgroup_law G (nat_to_group n0) (sgroup_law G r (group_inverse G r)));
auto with *.
apply Trans with (sgroup_law G (nat_to_group n0) (monoid_unit G));
auto with *.
apply Trans with (nat_to_group n0); auto with *.
apply
Trans
with (sgroup_law G (group_inverse G r) (sgroup_law G r (nat_to_group n0)));
auto with *.
apply
Trans
with (sgroup_law G (sgroup_law G (group_inverse G r) r) (nat_to_group n0));
auto with *.
apply Trans with (sgroup_law G (monoid_unit G) (nat_to_group n0));
auto with *.
Qed.
Hint Resolve nat_to_group_com3: algebra.

Lemma Zl5 : Equal (Z_to_group_nat_fun (ring_unit ZZ)) r.
unfold Z_to_group_nat_fun in |- *.