diff --git a/ex-4_45-xx.scm b/ex-4_45-54.scm
similarity index 90%
rename from ex-4_45-xx.scm
rename to ex-4_45-54.scm
index 0f2aaf6..c6512c5 100644
--- a/ex-4_45-xx.scm
+++ b/ex-4_45-54.scm
@@ -355,7 +355,7 @@
         (b (analyze (if-fail-second exp))))
     (lambda (env succeed fail)
       (a env
-         (lambda (a-value fail2) (succeed a-value fail))
+         (lambda (a-value fail2) (succeed a-value fail2))
          (lambda () (b env succeed fail))))))
 
 ;;; Amb-Eval input:
@@ -368,22 +368,55 @@
            (require (even? x))
            x)
          'all-odd)
-))
 
-(assert (first result) 'all-odd)
-
-(amball '(begin
 (if-fail (let ((x (amb 1 3 5 8)))
            (require (even? x))
            x)
          'all-odd)
+
 ))
 
-(assert (first result) 8)
-
+(assert result '(8 all-odd))
 
 (display "\nex-4.53\n")
 
+; (let ((pairs '()))
+;   (if-fail (let ((p (prime-sum-pair '(1 3 5 8) '(20 35 110))))
+;              (permanent-set! pairs (cons p pairs))
+;              (amb))
+;            pairs))
+
+; The code will yield all prime-sum-pairs as expected.
+
+(display "[answered]\n")
 
 (display "\nex-4.54\n")
 
+; Exercise 4.54.  If we had not realized that require could be implemented as
+; an ordinary procedure that uses amb, to be defined by the user as part of a
+; 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.
+
+(define (analyze-require exp)
+  (let ((pproc (analyze (require-predicate exp))))
+    (lambda (env succeed fail)
+      (pproc env
+             (lambda (pred-value fail2)
+               (if <??>
+                   <??>
+                   (succeed 'ok fail2)))
+             fail))))
+
+(display "[done]\n")
+
diff --git a/ex-4_55-xx.scm b/ex-4_55-xx.scm
new file mode 100644
index 0000000..d297ad0
--- /dev/null
+++ b/ex-4_55-xx.scm
@@ -0,0 +1,4 @@
+(load "util.scm")
+
+(display "\nex-4.55\n")
+