Compute the set of valid tokens that can occur starting in state {@code s}.
If {@code ctx} is null, the set of tokens will not include what can follow
the rule surrounding {@code s}. In other words, the set will be
restricted to tokens reachable staying within {@code s}'s rule.
Compute the set of valid tokens that can occur starting in state {@code s}. If {@code ctx} is null, the set of tokens will not include what can follow the rule surrounding {@code s}. In other words, the set will be restricted to tokens reachable staying within {@code s}'s rule.