1. Apparatus for effecting action in a subject system, the apparatus comprising: memory configured to store a model of the subject system, the model including: objects which represent objects within the subject system, each object in the model having associated therewith data which define which of at least two states the object is in at a given time, event definition rules, each event definition rule defining a number of actions which are required to be initiated in response to an event, wherein an event is a change in the state of an object, action definition rules, each action definition rule defining a number of objects of which a state change is required be initiated in response to the corresponding action, and object definition rules, each object definition rule defining a number of events that are required to be triggered in response to a change in state of the corresponding object, wherein all events and actions are constituted as objects, and at least one processor configured: to be responsive to the triggering of an event to examine the corresponding event definition rule to determine what actions are required be initiated, and to initiate those actions, to be responsive to the initiation of an action to examine the corresponding action definition rule in the model to determine what objects are required to be changed in state, and to change the states of those objects accordingly, to execute actions initiated in response to a single event directly in a single step, to execute composite actions initiated in response to an event defined in terms of sub-actions initiated in response to events one of a) directly and b) indirectly resulting from the execution of the composite action, and to be responsive to a change in state of an object, to examine the corresponding object definition rule in the model to determine what events are required to be triggered, and to trigger those events.