*
   * <p>The function is applied lazily, invoked when needed. This is necessary for the returned map
   * to be a view, but it means that the function will be applied many times for bulk operations
   * like {@link Map#containsValue} and {@code Map.toString()}. For this to perform well, {@code
   * function} should be fast. To avoid lazy evaluation when the returned map doesn't need to be a