Finish section 4.3

ex-4_45-54.scm

@@ -397,14 +397,6 @@
 ; nondeterministic program, we would have had to implement it as a special
 ; form. This would require syntax procedures
 
-(define (require? exp) (tagged-list? exp 'require))
-
-(define (require-predicate exp) (cadr exp))
-
-; and a new clause in the dispatch in analyze
-
-; ((require? exp) (analyze-require exp))
-
 ; as well the procedure analyze-require that handles require expressions.
 ; Complete the following definition of analyze-require.
 
@@ -413,8 +405,8 @@
     (lambda (env succeed fail)
       (pproc env
              (lambda (pred-value fail2)
-               (if <??>
-                   <??>
+               (if (not (true? pred-value))
+                   (fail2)
                    (succeed 'ok fail2)))
              fail))))