Implement till 2.76

util.scm

@@ -34,3 +34,14 @@
   (if (> low high)
       nil
       (cons low (enumerate-interval (+ low 1) high))))
+
+; Put and get functions. We could have implemented this via a list of
+; three-tuples, but I don't know how to create global variables yet so we just
+; use this code from SO. Doesn't look too complicated.
+; https://stackoverflow.com/questions/5499005/how-do-i-get-the-functions-put-and-get-in-sicp-scheme-exercise-2-78-and-on
+(define *op-table* (make-hash-table))
+(define (put op type proc)
+  (hash-table/put! *op-table* (list op type) proc))
+(define (get op type)
+  (hash-table/get *op-table* (list op type) #f))
+