code: @Pure
  	public final OctTreeZone zoneOf(N child) {
  		final int idx = indexOf(child);
  		final OctTreeZone[] zones = OctTreeZone.values();
  		if (idx < 0 || idx >= zones.length) {
  			return zones[idx];
  		}
  		return null;
  	}
docs: Replies the zone of the specified child.
  
  @param child the child.
  @return the zone or <code>null</code>.