private void advance() {
      while( true ) {
        _idx++;                 // Next index
        while( (_idx>>6) >= _nbsi2._bits.length ) { // Index out of range?
          if( _nbsi2._new == null ) { // New table?
            _idx = -2;          // No, so must be all done
            return;             //
          }
          _nbsi2 = _nbsi2._new; // Carry on, in the new table
        }
        if( _nbsi2.contains(_idx) ) return;
      }
    }
Standard Java {@link Iterator}.  Not very efficient because it
auto-boxes the returned values.
jctools-core/src/main/java/org/jctools/maps/NonBlockingSetInt.java