The Watcher Index Cell
The Petri net for the Watcher is shown below.
Before a transition is fired, it is necessary to test
whether certain predicate is satisfied. For example,
the index cell must be in state S0 to receive
message m1, and in state S1
to receive message m2.
This type of predicated Petri net is called
Evaluation Petri net (E-net).
E-net is very powerful and computationally equivalent to Turing machines.
