The semantics of the disambiguation constructs

In my piece about ambiguity, I mentioned the two constructs we have 
currently proposed for dealing with ambiguity, there notated as:

 !A and A!B.

The semantics of ! are as follows. In an alternative like

 A, !B, C

if B succeeds, then !B fails, and so the whole alternative fails; if B 
fails, then !B succeeds, consuming no input characters, and so the rest of 
the alternative starts at the same character position that !B started at.

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? 

Steven

Received on Monday, 2 February 2026 17:35:18 UTC