(defun frobnicate (foo bar &optional baz)
  "blabfldjflsdkf"
  (when foo (+ foo bar (* baz (or baz 1)))))

(fmakunbound 'frobnicate)

(defgeneric frobnicate (foo bar &optional baz)
  (:documentation "blabfldjflsdkf")
  (:method (foo bar &optional baz)
    (when foo (+ foo bar (* baz (or baz 1))))))