I'll give it a try, but we're actually trying to do this:

YYY && ! ('a' | 'b' | ZZZ) | 'something else'
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So shouldn't this be the syntax?

XXX: YYY ('a' | 'b' | ZZZ) <reject>
   | 'something' 'else'

YYY: 'some string'
ZZZ: 'ww' | 'xx' | 'yy' | 'zz'
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I'm not really sure if this is valid but I'll give it a try for now.

It's pretty confusing actually, because another Production I'd like to write is:

NonTerminator: SourceCharacter but not LineTerminator
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So this is my first attempt:

NonTerminator: SourceCharacter |
               SourceCharacter LineTerminator <reject>
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Anyone think this is correct? or is it:

NonTerminator: SourceCharacter LineTerminator <reject>
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somehow I do not think the second one is correct...

Does the <reject> apply to the entire line or just the item that it follows?