cktracking parser, which "forks" the stack when there are multiple productions
 which can be applied to a given stack/input configuration.
 This is adapted from alg 4.14. p. 187 in Compilers by Aho, Sethi, Ullman.
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<DL>
<DT><B>Since:</B><DD>Tue May 07 17:52:21 2002</DD>
</DD>
<DT><B>Author:</B><DD><a href=mailto:eric@metrotech-consulting.com>Eric Melz</a></DD>
</DD>
</DL>
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