public void addState(ATNState state) {
		if (state != null) {
			state.atn = this;
			state.stateNumber = states.size();
		}

		states.add(state);
	}
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