Enables "packrat" parsing, which adds memoizing to the parsing logic.
           Repeated parse attempts at the same string location (which happens
           often in many complex grammars) can immediately return a cached value,
           instead of re-executing parsing/validating code.  Memoizing is done of
           both valid results and parsing exceptions.

           This speedup may break existing programs that use parse actions that
           have side-effects.  For this reason, packrat parsing is disabled when
           you first import pyparsing.  To activate the packrat feature, your
           program must call the class method C{ParserElement.enablePackrat()}.  If
           your program uses C{psyco} to "compile as you go", you must call
           C{enablePackrat} before calling C{psyco.full()}.  If you do not do this,
           Python will crash.  For best results, call C{enablePackrat()} immediately
           after importing pyparsing.

def enablePackrat():
        """Enables "packrat" parsing, which adds memoizing to the parsing logic.
           Repeated parse attempts at the same string location (which happens
           often in many complex grammars) can immediately return a cached value,
           instead of re-executing parsing/validating code.  Memoizing is done of
           both valid results and parsing exceptions.

           This speedup may break existing programs that use parse actions that
           have side-effects.  For this reason, packrat parsing is disabled when
           you first import pyparsing.  To activate the packrat feature, your
           program must call the class method C{ParserElement.enablePackrat()}.  If
           your program uses C{psyco} to "compile as you go", you must call
           C{enablePackrat} before calling C{psyco.full()}.  If you do not do this,
           Python will crash.  For best results, call C{enablePackrat()} immediately
           after importing pyparsing.
        """
        if not ParserElement._packratEnabled:
            ParserElement._packratEnabled = True
            ParserElement._parse = ParserElement._parseCache