code: @Pure
  	public N getChildAt(QuadTreeZone zone) {
  		switch (zone) {
  		case NORTH_WEST:
  			return this.nNorthWest;
  		case NORTH_EAST:
  			return this.nNorthEast;
  		case SOUTH_WEST:
  			return this.nSouthWest;
  		case SOUTH_EAST:
  			return this.nSouthEast;
  		default:
  		}
  		return null;
  	}
docs: Replies the node that is corresponding to the given zone.
  
  @param zone the zone.
  @return the child node for the given zone, or <code>null</code>.