Reentrancy in feature structures

[Mike Maxwell]

John Beavers wrote:
> On Tue, 2 Jul 2002, Luis Casillas wrote:
>
>> 4. A tree satisfies an atomic path equivalence PATH1 = PATH2 iff the
>>    the subtree rooted at the node reached by following PATH1, and the
>>    subtree rooted at the node reached by following PATH2, are
>> identical.
>>
>> As far as I can see, reentrancy is just a way of guaranteeing that 4
>> will hold.  Am I missing something?
>
> Well, it seems to me that you've just replaced co-identification with
> some notion of equivalence, and I don't think this makes a
> difference.  It certainly wouldn't make any difference in whatever
> way you build such models (even the problem of infinite recursion is
> still an issue if a path is constrained to be equivalent to a
> superpath).  As far as the models themselves, well, any dag you get
> as a model can be converted into a tree algorithmically but you lose
> the equivalence information as part of the data struture.  It seems
> to me that's the only difference, and depending on whether you find
> that kind of information useful it could be an unimportant difference.

As a lurker on this list, let me ask a couple naive questions.  First,
it seems to me that the meaning of "identical" in Luis Casillas'
definition is a crucial issue here.  Dredging something out of my old
lisping days, I recall that Lisp made a distinction between (at least)
two notions of "identical", often labeled "eq" and "equal".  One (I
can't recall which) required identity of content (so e.g. two strings
were equal under this definition if they were "spelled" the same), while
the other required identity of reference (i.e. they were the same memory
location).  I presume that condition 4 requires the latter (stronger)
definition, correct?  In which case the equivalency really is just the
same as re-entrancy, correct?

Second (and assuming the answer to the first question is "yes"), I can
think of one obvious case where this sort of thing makes a difference,
namely with co-reference.  E.g. in some language where verbs are marked
for agreement with subject and object, you would get a reflexive form
only where the subject and object agreement features were identical in
the strong sense.  (Otherwise you couldn't distinguish the translation
equivalent of "He saw him" from "He saw himself.")

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

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