; Call expressions

(+ 1 2 3 4)
(+)
(*)
(- 12)
(- 20 1 2 3 4 5)
(* 2 (+ 1 (* 2 2 2 2 3 3 7)))
(number? 12)
(integer? 3.3)
(zero? 2)

; Definitions

(define (square x) (* x x))

(define (average x y) (/ (+ x y) 2))

(define (abs x)
  (if (< x 0)
      (- x)
      x))

(define (sqrt x)
  (define (improve guess)
    (average guess (/ x guess)))
  (define (sqrt-iter guess)
    (if (= (square guess) x)
        guess
        (sqrt-iter (improve guess))))
  (sqrt-iter 1))

; List demos

(cons 1 2)
(cons 1 (cons 2 nil))
(cons 1 (cons 2 (cons 3 4)))
(cons (cons 1 2) 2)
(cons (cons 1 2) nil)
(cons (cons 1 (cons 2 nil)) nil)
(cons (cons 1 2) (cons 3 nil))

(pair? (cons 1 2))
(pair? (cons 1 (cons 2 nil)))
(pair? nil)
(null? nil)
(null? (cons 1 2))

(list 1 2)
(list 1 2 3 4)
(cdr (list 1 2 3 4))

(define x (cons 1 2))
(list (car x) (cdr x))
(cons (car x) (cons (cdr x) nil))

(define (length items)
  (if (null? items)
      0
      (+ 1 (length (cdr items)))))

(define squares (list 1 4 9 16 25))

(length squares)