Section ExtremalClass.
Variables (gT : finGroupType) (G : {group gT}).
Inductive extremal_group_type :=
ModularGroup | Dihedral | SemiDihedral | Quaternion | NotExtremal.
Definition index_extremal_group_type c :=
match c with
| ModularGroup => 0
| Dihedral => 1
| SemiDihedral => 2
| Quaternion => 3
| NotExtremal => 4
end%N.
Definition enum_extremal_groups :=
[:: ModularGroup; Dihedral; SemiDihedral; Quaternion].
End ExtremalClass.

(* This is the second part of Aschbacher, exercise (8.4). *)
Lemma prime_Ohm1P gT p (G : {group gT}) :
p.-group G -> G :!=: 1 ->
reflect (#|'Ohm_1(G)| = p)
(cyclic G || (p == 2) && (extremal_class G == Quaternion)).