End ExtremalStructure.
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].

Lemma semidihedral_classP :
extremal_class G = SemiDihedral <-> (exists2 n, n > 3 & G \isog 'SD_(2 ^ n)).