Activator
SDL differentiates between process definition and
process instance
Defined process activation mechanism
Receiver process instance
   (1 receiverprocess start
     ((ackid . nil) (frameid . nil) (self . 1)
        (sender . nil) (parent . 0) (offspring . nil)
    ((start …)) nil)
Correctness property: defined a recognizer for
valid instances of a system
   (defthm activate-makes-instance
     (implies (wf-type S)
             (wf-instance (activate S) S)))