Javadoc
Given an NFA state number, how many times has the NFA-to-DFA
conversion pushed that state on the stack? In other words,
the NFA state must be a rule invocation state and this method
tells you how many times you've been to this state. If none,
then you have not called the target rule from this state before
(though another NFA state could have called that target rule).
If n=1, then you've been to this state before during this
DFA construction and are going to invoke that rule again.
Note that many NFA states can invoke rule r, but we ignore recursion
unless you hit the same rule invocation state again.