I want to add "categories" inside of a category. An Example would be:
-Fuel
-----Mode 1
---------Gear 1
---------Gear 2
---------Gear N
-----Mode 2
---------Gear 1
---------Gear 2
---------Gear N
-Spark
-----Mode 1
---------Gear 1
---------Gear 2
---------Gear N
-----Mode 2
---------Gear 1
---------Gear 2
---------Gear N
Any helpful hints on how to do it?