Implement up to 2.41

util.scm

@@ -12,6 +12,25 @@
 
 (define (average a b) (/ (+ a b) 2.0))
 (define (id n) n)
+(define identity id)
 (define (inc n) (+ n 1))
 (define nil '())
+(define (divides? a b) (= (remainder b a) 0))
+(define (cube n) (* n n n))
+(define (even? n) (= (remainder n 2) 0))
+(define (odd? n) (= (remainder n 2) 1))
 
+; copied prime? from 1.21
+(define (find-divisor n test-divisor)
+  (cond ((> (square test-divisor) n) n)
+        ((divides? test-divisor n) test-divisor)
+        (else (find-divisor n (+ test-divisor 1)))))
+(define (smallest-divisor n)
+  (find-divisor n 2))
+(define (prime? n) (if (= n 1) #f (= n (smallest-divisor n))))
+
+; https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book-Z-H-15.html
+(define (enumerate-interval low high)
+  (if (> low high)
+      nil
+      (cons low (enumerate-interval (+ low 1) high))))