update-instance-structure

[Gregor.pa@Xerox.COM]

There are a couple of minor problems with the update-instance-structure
section in chapter 2.

In the arguments section, the first sentence where it says "When
make-instances-obsolete is invoked..".  It probably means something more
like "After make-instances-obsolete is invoked, of after a class has
been redefined, all the instances must be updated.  When an instance is
being updated ...".

In the examples section.  I believe the defmethod of
update-instance-structure should be at the end of the example.  Even
though this code might work interpreted (in some implementations) it
definitely won't compile.  This is because setf of position-rho is used
before position-rho is defined.
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