Add exercise 4.28

mceval/4-28.txt

@@ -0,0 +1,13 @@
+Exercise 4.28: The actual-value call is necessary in the case where
+the procedure to be applied is itself returned from another procudure.
+
+Contrived example:
+
+(define (choose-proc p1 p2) p1)
+(define (f x) (+ x 1))
+(define (g x) (+ x 4))
+
+((choose-proc f g) 3)
+
+Without the force-it call, we would have a thunk in the first position
+of the expression, which would not work.