Each package has a list of associated implementation packages. A
locked package, and the symbols whose home package it is, can be
modified without violating package locks only when *package*
is
bound to one of the implementation packages of the locked package.
Unless explicitly altered by defpackage
,
sb-ext:add-implementation-package
, or
sb-ext:remove-implementation-package
each package is its own
(only) implementation package.