(defun fun (a b c &optional d e &rest f) 
  (mapc 'princ (list "Required: a: " a " b: " b " c: " c "\n"))
  (mapc 'princ (list "Optional: d: " d " e: " e "\n"))
  (mapc 'princ (list "Rest    : f: " f "\n"))
  t)
fun

(fun 1 2 3)
Required: a: 1 b: 2 c: 3
Optional: d: nil e: nil
Rest    : f: nil
t


(fun 1 2 3 4)
Required: a: 1 b: 2 c: 3
Optional: d: 4 e: nil
Rest    : f: nil
t

(fun 1 2 3 4 5 6 7 8 9)
Required: a: 1 b: 2 c: 3
Optional: d: 4 e: 5
Rest    : f: (6 7 8 9)
t

(defsubst abs (x)
  (if (>= x 0) x (- x)))
abs

(abs 3)
3
(abs -4)
4

(defun difference (n m)
  (abs (- n m)))
difference

(difference 6 9)
3

(symbol-function 'difference)
(lambda (n m) (abs (- n m)))

(defun move-line (L)
  (interactive "nNr of lines:")
  (forward-line L))
move-line