code: func (nt *NodeTree) removeZone(zone string) {
  	delete(nt.tree, zone)
  	for i, z := range nt.zones {
  		if z == zone {
  			nt.zones = append(nt.zones[:i], nt.zones[i+1:]...)
  			return
  		}
  	}
  }
docs: // removeZone removes a zone from tree.
  // This function must be called while writer locks are hold.