public int defineDecisionState(DecisionState s) {
		decisionToState.add(s);
		s.decision = decisionToState.size()-1;
		return s.decision;
	}
Compute the set of valid tokens that can occur starting in {@code s} and
staying in same rule. {@link Token#EPSILON} is in set if we reach end of
rule.
runtime/Java/src/org/antlr/v4/runtime/atn/ATN.java