Packages and shadowing

[Richard M. Stallman <RMS at MIT-AI>]

The feature of warning about redefining a function in another package
is turned off for GLOBAL.  There is a feature whereby a package can
specify which symbols it wants to redefine in its superior package,
but nobody wanted to put the list of all system functions and
variables into the definition of SI and COMPILER, etc., so the warning
feature was turned off for symbols in GLOBAL

Perhaps the right thing to do is restrict the blanket exemption for
redefining symbols in GLOBAL so that it only applies if the current
package is a subpackage of SYSTEM.