1. Specifies the mapping of each user action on a widget to one of the ICO's user services.
    2. Act: (Widget * Event)  =>  Services
    1. Each service -- related to at least one transition; only available when at least one related transition is fireable.
    2. Avail: Service => P(T); P(T) is the power set of Transitions.
For every s belonging to Services, Avail(s) <> Empty
For every s and s' belonging to Services, Avail(s) and Avail(s') have no interception.