[developers] LKB vs. PET divergences
Ann.Copestake at cl.cam.ac.uk
Mon Feb 14 15:41:10 CET 2005
I don't have PET here, but what seems to be going on is a discrepancy in the
way that the glb is being computed or a discrepancy in the way that
the rule is being treated.
vstem-vstem-rule is defined as
vstem-vstem-rule := head-final-type &
the LKB gives glbtype71 as the glb of those two. This is a subtype of
argument-binding-princ which brings in the coindexation between the HOOK of
the mother and the HOOK of the HEAD-DTR. head-final-type sets the HEAD-DTR to
the second DTR. So the coindexation is as expected if glbtype71 is the type.
Should glbtype71 inherit from argument-binding-princ? Well, I'd say it's
plausible, given that the glbtypes are supposed to minimize the hierarchy.
head-final-type has four immediate subtypes: head-specifier-rule-type,
head-complement-hf-type and head-subject-rule. These are all subtypes of
orth-princ, but they are also all subtypes of argument-binding-princ.
Possibly, PET is creating a new `pure' glbtype at the point when the
definition of vstem-vstem-rule is read in. This would be incorrect - as
stated in the previous message, the formalism says that this is just a
description of a feature structure. We don't allow the calculation of new
glbs during unification in general and it shouldn't be allowed here.
Alternatively, we have a discrepancy in the glb calculation algorithm, which
would also be possible, though horrible to fix. If there's a formal
definition of glbtype introduction which says whether it is or is not supposed
to introduce pure types in this situation, then we can decide which behaviour
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