I have a production which has some negative logic in it... Basically, how do we define XXX so that it is YYY but not 'a' or 'b' or ZZZ, OR 'something else'? I've provided some pseudo-code for this, but can't get further than what's below:

XXX: YYY                   # BUT NOT 'a' | 'this string' | ZZZ
     | 'something' 'else'

YYY: 'some string'
ZZZ: 'ww' | 'xx' | 'yy' | 'zz'
[download]

I'm wondering if the gods of Parse::RecDescent know how I can implement this....

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