code: @Override
      public V replace(K key, V value) {
          return rMap.replace(key, value);
      }
docs: Replaces the entry for a key only if currently mapped to some value.
  This is equivalent to
  <pre>
  if (map.containsKey(key)) {
  return map.put(key, value);
  } else return null;</pre>
  except that the action is performed atomically.
  
  @param key key with which the specified value is associated
  @param value value to be associated with the specified key
  @return the previous value associated with the specified key, or
  <tt>null</tt> if there was no mapping for the key.
  (A <tt>null</tt> return can also indicate that the map
  previously associated <tt>null</tt> with the key,
  if the implementation supports null values.)
  @throws UnsupportedOperationException if the <tt>put</tt> operation
  is not supported by this map
  @throws ClassCastException if the class of the specified key or value
  prevents it from being stored in this map
  @throws NullPointerException if the specified key or value is null,
  and this map does not permit null keys or values
  @throws IllegalArgumentException if some property of the specified key
  or value prevents it from being stored in this map