A NullToken is a marker class used to break deadlock in certain
topologies of DDE models. A NullToken object does not represent
modeled computation but serves as an indicator that an actor in
a DDE model can safely advance time to be equal to the time stamp
associated with the NullToken.
NullTokens are not part of the Ptolemy II type lattice. As such,
a NullToken will violate type resolution if passed through a
typed IO port. For this reason, Null Tokens are placed directly
into receivers without using the send() method of TypedIOPort.
This tactic circumvents type resolution constraints and affirms
the notion that NullTokens do not represent computation and hence
should never be incorporated into actor code. Note further, that
the get() method of DDEReceiver does not return NullTokens but
instead consumes them and then proceeds until a "real" token
becomes available.