Dear all,
I have a problem understanding how subsumption and constraining equations work together in the XLE. As far as I can see, XLE's behavior is different from what the documentation says, but I am not sure I understand.
The basic problem is this: I want to allow nominalizations of S's that have a CASE value but not other S's. The rule I use is this:
NP --> { N' | S: ! << ^ (! CASE) }
So an NP can expand to S, in which case the S's f-structure should subsume that of the NP, so all the information gets passed up from the f-structure of the S to the f-structure of the NP. Then there is a constraining/existential equation requiring the S's f-structure to have CASE - or so I thought. It wouldn't be enough that the NP gets assigned CASE from above, so to speak.
If I read it correctly, he documentation of explicit subsumption says that it behaves differently from implicit subsumption (in sets) in that constraining equations don't get distributed. But as it happens, XLE accepts a solution where S in itself has no case, but the NP gets case from the verb. So even though it displays two different f-structures for the NP and the S (since they differ in CASE), it allows for the existential constraint to be satisfied in the "wrong" f-structure.
I can achieve the effect I want by adding @(COMPLETE(! CASE)), forcing local satisfaction of the existential constraint. In that case, there is no need to use subsumption at all, of course. So implementation-wise I am ok, but I am interested in subsumption from a theoretical point of view as well, so if anyone can help explain XLE's behavior and whether there is a particular theoretical reason for it, I'd be grateful. I have a minimal implementation which recreates the problem, if anyone is interested.
Best, Dag Haug