EXPORT after USE-PACKAGE

[David.Dill@CMU-CS-A (L170DD60)]

It seems to me that the issue that has caused the most grief, directly
or indirectly, in the design of the package system has been what to
do when a package decides to change its export list after it has been
used by another package.  I have been operating under the assumption
that the only legitimate reason to do this is during debugging or
development, to recover from a mistake in setting up a package or
to "remove the back" from a package in order to poke around inside
conveniently.  Does everyone else believe this (Moon in particular)?

If this is the case, then saying so in the manual could simplify some
of the explanations of lookup rules, and also allow more flexibility
in individual implementations (e.g. copy vs. search rules).