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    Date: Thu, 17 Apr 86 08:08:14 PST
    From: David J. Marchette <marchett%cod@nosc.ARPA>

    Is there an easy way to tell if an instance variable (or slot) is available
    in a particular structure?  

What good would it do to have a SLOTP predicate? What operation can you do
on a structure once you know it has such a slot? The point is that you're
not supposed to go around looking for slots by name except maybe in a debugger,
and debuggers are intended to be provided in an implementation-dependent fashion
by each system. 

It's a data-abstraction violation to know what a structure calls its slot names.
If you're supposed to be poking about in the structure, the person who authorized 
you to do so will have given you the following information: a symbol which can 
be used as a second argument to TYPEP to tell if you have one of the structures
in question, a function which creates such structures and/or a set of functions
which will access their slots, and documentation on how to use those accessors