CMUCL provides two types of package locks, as an extension to the
ANSI Common Lisp standard. The package-lock protects a package from
changes in its structure (the set of exported symbols, its use list,
etc). The package-definition-lock protects the symbols in the package
from being redefined due to the execution of a defun,
defmacro, defstruct, deftype or defclass
form.