function-names

(defun function-names (fun)
  "
RETURN: A list of function names fbound to the function FUN.
NOTE:   We only scan symbols and (setf symbols).  Functions could also
        be bound to slots or to various other lisp objects.
"
  (let ((names '()))
    (dolist (pack (list-all-packages) (delete-duplicates names))
      (do-symbols (sym pack)
        (when (and (fboundp sym) (eql fun (symbol-function sym)))
          (push sym names))
        (let ((setter `(setf ,sym)))
          (when (and (fboundp setter) (eql fun (fdefinition setter)))
            (push setter names)))))))