--- a/gamelib/products.py	Sun May 06 20:54:03 2012 +0200
+++ b/gamelib/products.py	Sun May 06 20:57:04 2012 +0200
@@ -1,14 +1,22 @@
 from gamelib import research
 
 
+# Kinds of product.
+HAND_WEAPON = 'hand weapon'
+VEHICLE = 'vehicle'
+DOOMSDAY_DEVICE = 'doomsday device'
+PATHOGEN = 'pathogen'
+
+
 class Product(object):
     NAME = None
     PREREQUISITES = ()
     ACQUISITION_CHANCE = 0.8
-    COST = 0
+    COST = None
     UPGRADE_REQUIREMENT = 1
     STARTING_PRODUCT = False
     SCIENCE_TYPE = 'product'
+    CATEGORIES = ()
 
     def __init__(self, points=0):
         self.points = points
@@ -24,6 +32,7 @@
 class MachineGun(Product):
     NAME = "Machine gun"
     COST = 100
+    CATEGORIES = (HAND_WEAPON,)
     STARTING_PRODUCT = True
 
     def __init__(self, points=0):
@@ -38,14 +47,17 @@
 
 class LightningGun(Product):
     NAME = "Lightning gun"
+    COST = 300
+    CATEGORIES = (HAND_WEAPON,)
     PREREQUISITES = (
         (research.Tesla, 1),
         )
-    COST = 300
 
 
 class TeslaTank(Product):
     NAME = "Tesla tank"
+    COST = 1000
+    CATEGORIES = (VEHICLE,)
     PREREQUISITES = (
         (research.Tesla, 3),
         )
@@ -53,7 +65,8 @@
 
 class DoomsdayVirus(Product):
     NAME = "Doomsday virus"
+    COST = 100000
+    CATEGORIES = (DOOMSDAY_DEVICE, PATHOGEN)
     PREREQUISITES = (
         (research.Biogenetics, 5),
         )
-    COST = 1000