private void print( int d, String msg ) {
      for( int i=0; i<d; i++ )
        System.out.print("  ");
      System.out.println(msg);
    }
Standard Java {@link Iterator}.  Not very efficient because it
auto-boxes the returned values.
jctools-core/src/main/java/org/jctools/maps/NonBlockingSetInt.java