Re: The semantics of the disambiguation constructs

In response to my action:

2026-02-03-a: JWL to study SP’s subtraction operator proposal

On 02/02/2026 17:35, Steven Pemberton wrote:
> My question, what are the proposed semantics of A!B (or A - B if you 
> like)? For instance, is there are a requirement that A and B span the 
> same character positions? Does B have to be a subset of A, or is the 
> exclusion of the intersection of the two implied? 

The 'subtraction' operator I implemented (*A¬B*)  was perhaps more 
restricted and designed to solve a specific problem in 'reserved 
keywords' in the XPath grammars.

It was defined to match a term *A* (that could be any tree of 
productions) /unless/ *B* (which could also be any tree of 
productions) matched _across the same input character range_, e.g.

      FunctionCall: FunctionName, -"("...
      FunctionName: NCName ¬ ReservedNames.
    ReservedNames: "item";"type";"element"...

In effect is was subtracting the second set from the first set of 
potential matches. Unless *B* was a subset of *A* (or *A* and *B* have a 
potentially overlapping intersection), it reduces to just the first 
term. As such it does require some mechanism to look down both paths, 
which I implemented in my Earley parser by some cross-linking between 
two paths being explored simultaneously (as if they were alternatives) 
and failing the first match (assuming it succeeds) if the second 
succeeds at the same end position. So in the example above:

    elem() & elementary()

would succeed, but

    element()

would fail.

I agree that this is not as powerful as the proposed negation/lookahead 
failure being considered, but it is a very common use case.

-- 
*John Lumley* MA PhD CEng FIEE
john@saxonica.com