Index: engine.py
===================================================================
--- engine.py	(revision 75545)
+++ engine.py	(working copy)
@@ -132,7 +132,12 @@
         bjam_flags = reduce(operator.or_,
                             (action_modifiers[flag] for flag in flags), 0)
 
-        bjam_interface.define_action(action_name, command, bound_list, bjam_flags)
+        # We allow command to be empty so that we can define 'action' as pure
+        # python function that would do some conditional logic and then relay
+        # to other actions.
+        assert command or function
+        if command:
+            bjam_interface.define_action(action_name, command, bound_list, bjam_flags)
 
         self.actions[action_name] = BjamAction(action_name, function)