Redefining structures

[Drew McDermott <Mcdermott@YALE.ARPA>]

The way STYPE etc. are currently defined, if you redefine a structure 
type it is verboten to access anything built using the old definition.
This occasionally forces me to reload a large program.  In many cases
the structure type has not really been redefined; I just reloaded
an old file for some reason, and redefined the structure in it.  In
other cases the redefinition adds a slot or something to the structure.

I can see why strict purity would force one to make things work this
way; the identity between the old and the new definitions of the
"same" concept is in the mind of the programmer only.  However, for
practical purposes during debugging, the system ought to do better
at guessing what you mean.  I would not object to a warning message
the first time an object is referenced in a bogus way.
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