Token Passing Continuation
Insures that each message in the expression is
sent after the previous message has been
processed. It also allows that the return value of
one message invocation may be used as an
argument for a later invocation in the
expression.
Example:
a1<-m1() @ a2<-m2( token );
Send m1 to a1 and then after m1 finishes, send the result with
m2 to a2.