// An IMap<K,V> is an object of any class that implements IMap<K,V>.
      //
      // Interpretation: An IMap<K,V> represents an immutable finite function
      // from keys to values.
      //
      // IMap<K,V> extends java.util.Map<K,V>, so an IMap<K,V> can be used
      // as an unmodifiable Map<K,V>.
      
      import java.util.Map;
      
      interface IMap<K,V> extends Map<K,V> {
      
          // Returns an IMap like this one except the given key is associated
          // with the given value.  The extend operation does not modify this
          // IMap.
      
          IMap<K,V> extend (K key, V value);
      }