Add some example code from second chapter

chapter2.scm

@@ -0,0 +1,77 @@
+#lang sicp
+(define (gcd a b)
+  (if (= b 0)
+      a
+      (gcd b (remainder a b))))
+
+(define (add-rat x y)
+  (make-rat (+ (* (numer x) (denom y))
+               (* (numer y) (denom x)))
+            (* (denom x) (denom y))))
+
+(define (sub-rat x y)
+  (make-rat (- (* (numer x) (denom y))
+               (* (numer y) (denom x)))
+            (* (denom x) (denom y))))
+
+(define (mul-rat x y)
+  (make-rat (* (numer x) (numer y))
+            (* (denom x) (denom y))))
+
+(define (div-rat x y)
+  (make-rat (* (numer x) (denom y))
+            (* (denom x) (numer y))))
+
+(define (equal-rat? x y)
+  (= (* (numer x) (denom y))
+     (* (numer y) (denom x))))
+
+(define (make-rat n d)
+  (let ((g (gcd n d)))
+    (cons (/ n g)
+          (/ d g))))
+(define (numer x) (car x))
+(define (denom x) (cdr x))
+
+(define (print-rat x)
+  (newline)
+  (display (numer x))
+  (display "/")
+  (display (denom x)))
+
+(define one-half (make-rat 1 2))
+(define one-third (make-rat 1 3))
+(define one-through-four (list 1 2 3 4))
+
+(define (list-ref items n)
+  (if (= n 0)
+      (car items)
+      (list-ref (cdr items)
+                (- n 1))))
+
+;(define (length items)
+;  (if (null? items)
+;      0
+;      (+ 1 (length (cdr items)))))
+
+(define (length items)
+  (define (length-iter a count)
+    (if (null? a)
+        count
+        (length-iter (cdr a)
+                     (+ 1 count))))
+  (length-iter items 0))
+
+(define (append list1 list2)
+  (if (null? list1)
+      list2
+      (cons (car list1)
+            (append (cdr list1)
+                    list2))))
+
+(define (scale-list items factor)
+  (if (null? items)
+      nil
+      (cons (* (car items) factor)
+            (scale-list (cdr items)
+                        factor))))