*applyhook* is a system variable that exists and is initialized to NIL. It is a hook that is intended to contain a user function that is to be called whenever a function is applied to a list of arguments. It is not, however, implemented in XLISP 2.0, it only exists as a dummy hook.
*applyhook* ; returns NIL
Common Lisp: *applyhook* is defined in Common Lisp and is often used to implement function stepping functionality in a debugger.