public TopDownBuilder add(final Layer layer){
  if (bottom != null) {
    bottom.setLowerLayer(layer);
  }
  stack.add(layer);
  bottom=layer;
  return this;
}

public TopDownBuilder add ( final Layer layer ) { if ( bottom != null ) { bottom . setLowerLayer ( layer ) ; } stack . add ( layer ) ; bottom = layer ; return this ; }
adds the specified layer below the currently lowest layer .
