diff --git a/scheme/httpd/surflets/web-server/root/surflets/calculate.scm b/scheme/httpd/surflets/web-server/root/surflets/calculate.scm
index a09311f..0c7ab56 100644
--- a/scheme/httpd/surflets/web-server/root/surflets/calculate.scm
+++ b/scheme/httpd/surflets/web-server/root/surflets/calculate.scm
@@ -7,7 +7,7 @@
 	scheme)
   (begin
 
-    ;; This uses callbacks.
+    ;; This doesn't use callbacks anymore.
     
     (define *operator-alist*
 	    `(("+" . ,+)
@@ -33,15 +33,6 @@
 			   `(option ,(operator-symbol operator)))
 			 *operator-alist*)))))
 
-    (define (change-operator-callback)
-      (make-callback 
-       (lambda (req)
-	 (change-operator
-	  ;; This yields an error only when the browser doing wrong.
-	  (input-field-value operator-input-field
-			     (get-bindings req))))
-       ))
-
 
     (define (make-number-input-field/default default)
       (if default
@@ -52,6 +43,8 @@
       (let* ((update-text (:optional maybe-update-text ""))
 	     (number-field1 (make-number-input-field/default number1))
 	     (number-field2 (make-number-input-field/default number2))
+	     (calculate-button (make-submit-button "Calculate"))
+	     (change-button (make-submit-button "Change operator"))
 	     (req
 	      (send-html/suspend
 	       (lambda (new-url)
@@ -67,34 +60,42 @@
 			       (td ,(operator-symbol operator-pair))
 			       (td ,number-field2)
 			       (td " = ")
-			       (td ,(make-submit-button '(@ (value "calculate")))))))
-			 (hr)
-			 (p "You may choose another operator:")
-			 (servlet-form
-			  ,(change-operator-callback)
+			       (td ,calculate-button)))
+			  (hr)
+			  (p "You may choose another operator:")
 			  (table
 			   (tr (td ,operator-input-field)
-			       (td ,(make-submit-button 
-				     '(@ (value "change operator"))))))))))))
+			       (td ,change-button)))))))))
 	     (bindings (get-bindings req)))
-	(let ((number1 
-	       (with-fatal-error-handler 
-		(lambda (c d) #f)
-		(input-field-value number-field1 bindings)))
-	      (number2 
-	       (with-fatal-error-handler 
-		(lambda (c d) #f)
-		(input-field-value number-field2 bindings))))
-	  (if number1
-	      (if number2
-		  (calculate operator-pair number1 number2)
-		  (show-page operator-pair number1 number2 "Please enter a valid second number."))
-	      (show-page operator-pair number1 number2 "Please enter a valid first number."))
-	  )))
+	(cond
+	 ((input-field-binding calculate-button bindings)
+	  (let ((number1 (with-fatal-error-handler 
+			  (lambda (c d) #f)
+			  (input-field-value number-field1 bindings)))
+		(number2 (with-fatal-error-handler 
+			  (lambda (c d) #f)
+			  (input-field-value number-field2 bindings))))
+	    (if number1
+		(if number2
+		    (calculate operator-pair number1 number2)
+		    (show-page operator-pair number1 number2 
+			       "Please enter a valid second number."))
+		(show-page operator-pair number1 number2 
+			   "Please enter a valid first number."))))
+	 ((input-field-binding change-button bindings)
+	  (with-fatal-error-handler
+	   (lambda (c d)
+	     ;; This should never happen.
+	     (show-page operator-pair #f #f
+			"Internal error. Please retry or report."))
+	   (show-page (input-field-value operator-input-field
+					 bindings)
+		      #f #f)))
+	 (else
+	  ;; This should never happen.
+	  (show-page operator-pair #f #f
+		     "Internal error. Please retry or report.")))))
 		     
-    (define (change-operator to-operation)
-      (show-page to-operation #f #f))
-
     (define (calculate operator-pair number1 number2)
       (let ((operator (operator-operator operator-pair)))
 	(show-result number1 (operator-symbol operator-pair) number2