Reentrancy in feature structures

[Mike Maxwell]

I wrote:
> Are there other cases where the strong notion of identity is required,
> besides (person) co-reference?

To which Carl Pollard replied:
> I hope not, since in my current research I have foresaken the
> graphical implementation of feature structures in favor of a treatment
> in terms of higher order logic. In the models this means feature
> structure types are indexed product types and the feature structures
> themselves are members of those types.  Thus there is only plain old
> equality (EQ), nothing corresponding to EQUAL. This makes some things
> (such as formalizing set values and distinguishing neutralization from
> ambiguity) much easier, but also means that coindexing cannot be
> handled the same way as in HPSG.

OK, I'll bite: how do you do coindexing?  With s.t. like Luis Casiallas'
indices (i, j...)?  I used to think of those as a notation for
re-entrancy, but maybe not.  Is there a paper I can read that treats
(co)indexing under this treatment?

     Mike Maxwell
     Linguistic Data Consortium
     maxwell at ldc.upenn.edu