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From an NFA state, s, find the set of all labels reachable from s.
Used to compute follow sets for error recovery. Never computes
a FOLLOW operation. FIRST stops at end of rules, returning EOR, unless
invoked from another rule. I.e., routine properly handles
a : b A ;
where b is nullable.
We record with EOR_TOKEN_TYPE if we hit the end of a rule so we can
know at runtime (when these sets are used) to start walking up the
follow chain to compute the real, correct follow set (as opposed to
the FOLLOW, which is a superset).
This routine will only be used on parser and tree parser grammars.