The waitfor property is used to wait for the reception of a token
before sending a message. The property can add variable continuation restrictions dynamically,
enabling a flexible and-barrier for concurrent execution.
The following example shows that the message compare(b) can be delivered to Actor a if
Actors a and b have migrated to the same theater:
token x = a<-migrate("rmsp://europa.cs.rpi.edu:4040");
token y = b<-migrate("rmsp://europa.cs.rpi.edu:4040");
a<-compare(b) : waitfor(x,y);
