public IntervalSet nextTokens(ATNState s) {
        if ( s.nextTokenWithinRule != null ) return s.nextTokenWithinRule;
        s.nextTokenWithinRule = nextTokens(s, null);
        s.nextTokenWithinRule.setReadonly(true);
        return s.nextTokenWithinRule;
    }
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