public DecisionState getDecisionState(int decision) {
        if ( !decisionToState.isEmpty() ) {
            return decisionToState.get(decision);
        }
        return null;
    }
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