Actor is meant as an abstract class, i.e. no instances of Actor
should exist. An actor is associated with a shadow object. The shadow
acts like a normal object in the users address space, data and
processing that is done by the shadow may be stored and processed in a
remote process, the actor resides in this remote address space. The
actor and the shadow both have a channel, a communication port. This
allows the two to communicate with each other.
int tag