method combinations changing

[Gregor.pa@Xerox.COM]

As near as I can tell, the spec doesn't say enough about long form
method combinations to make it possible to do any caching of method
lookups in the face of them.

Specifically, it doesn't say something like the result returned by the
long form method combination must be a function of only the list of
applicable methods.  Not the time of day or anything like that.

Also, we have to deal with restrictions on changing a method combination
once it is defined.  As part of Common Lisp, you would think that the
results of any changes are undefined.  But, parts of CLOS say what
happens.  I think we need a restriction to prevent redefining a method
combination.
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