diff --git a/ex-5_01-06.scm b/ex-5_01-06.scm
index 1bfc43c..c9deb40 100644
--- a/ex-5_01-06.scm
+++ b/ex-5_01-06.scm
@@ -119,11 +119,11 @@
         (goto (label expt-loop))
       after-expt
         (restore continue)
-        (assign val (op *) (reg b) (reg val))   ; val now contains n(n - 1)!
-        (goto (reg continue))                   ; return to caller
+        (assign val (op *) (reg b) (reg val))
+        (goto (reg continue))
       base-case
-        (assign val (const 1))                  ; base case: 1! = 1
-        (goto (reg continue))                   ; return to caller
+        (assign val (const 1))
+        (goto (reg continue))
       expt-done)))
 
 (set-register-contents! expt-rec-machine 'b 3)
diff --git a/ex-5_07-xx.scm b/ex-5_07-xx.scm
index af52201..e7e8cee 100644
--- a/ex-5_07-xx.scm
+++ b/ex-5_07-xx.scm
@@ -1,8 +1,59 @@
 (load "util.scm")
 (load "misc/sicp-regsim.scm")
 
+(display "\nex-5.7 - test-machines\n")
 
-(display "\nex-5.7\n")
+; We have already tested the machines in the previous exercises.
 
+(display "[answered]\n")
 
-(display "\nex-5.8\n")
+; continue at 5.2.2 - the assembler
+
+(display "\nex-5.8 - double-here\n")
+
+(define (extract-labels text receive)
+  (define (warn-if-label-exists label labels)
+    (if (assoc label labels)
+      (begin
+        (display "WARNING duplicated label -- ")
+        (display label)
+        (newline))))
+  (if (null? text)
+      (receive '() '())
+      (extract-labels (cdr text)
+       (lambda (insts labels)
+         (let ((next-inst (car text)))
+           (if (symbol? next-inst)
+             (warn-if-label-exists next-inst labels))
+           (if (symbol? next-inst)
+               (receive insts
+                        (cons (make-label-entry next-inst
+                                                insts)
+                              labels))
+               (receive (cons (make-instruction next-inst)
+                              insts)
+                        labels)))))))
+
+(define double-here-machine
+  (make-machine
+    '(a)
+    '()
+    '(start
+        (goto (label here))
+      here
+        (assign a (const 3))
+        (goto (label there))
+      here
+        (assign a (const 4))
+        (goto (label there))
+      there)))
+
+(start double-here-machine)
+(assert (get-register-contents double-here-machine 'a) 3)
+
+; The register contains 3 because the assembler jumps to the first label in the
+; list.
+
+(display "\nex-5.9\n")
+
+(display "\nex-5.10\n")