diff -r 3abae898011d -r 40cabcdcd2be samples/openfixed/src/org/apidesign/openfixed/Calculator.java
--- a/samples/openfixed/src/org/apidesign/openfixed/Calculator.java	Sun Mar 20 20:52:33 2011 +0100
+++ b/samples/openfixed/src/org/apidesign/openfixed/Calculator.java	Thu Oct 30 21:30:10 2014 +0100
@@ -13,6 +13,17 @@
     private Calculator(EventSupport listeners) {
         this.listeners = listeners;
     }
+
+    /** An abstraction over various types of event delivery
+     * to listeners. Comes with four different implementations.
+     * A trivial one, asynchronous one, one with support for
+     * pending events and one for a batch events delivery.
+     */
+    interface EventSupport {
+        public void fireModificationEvent(ModificationEvent ev);
+        public void add(ModificationListener l);
+        public void remove(ModificationListener l);
+    }
     
     public static Calculator create() {
         return new Calculator(new TrivialEventSupport());