Re: The semantics of the disambiguation constructs

On 02/03/2026 16:09, Bethan Tovey-Walsh wrote:
> Let's take this grammar fragment:
>
>  A: ["a"-"z"]*.
>  B: "cat" ; "bat" ; "rat".
>  C: A ¬ B.
>
> I think I understand your view of the semantics to be this:
>
>  C is an A, unless the entirety of C also matches B, in which case it is a B

No - it's effectively:

    C is an A, unless the entirety of C also matches B, in which case not a C

> So if we had the input "caterpillar", we'd get:
>
>  <C>
>       <A>caterpillar</A>
>  </C>
Yes
> and if we had "cat", we'd get:
>
>  <C>
>       <B>cat</B>
>  </C>
No - this would fail, as in the example where element() failed
> and if we had "", we'd get:
>
>  <C>
>       <A/>
>  </C>
Yes, as B would not (yet) have succeeded when A did at the end of input.
> So, in the rule
>
>  C: A ¬ B.
>
> we have something rather like
>
>  C: A | B.
>
> in that C, if it matches, can be either an A or a B. The ¬ operator is simply a way to indicate that it cannot be*both* A and B.
No - it is more like a set-reduction (difference) operator.
-- 
*John Lumley* MA PhD CEng FIEE
john@saxonica.com