code: public function compareChildren($a, $b)
      {
          if ($this->_children[$a]->getTotalSize() > $this->_children[$b]->getTotalSize()) {
              $node_a              = $this->_children[$a];
              $node_b              = $this->_children[$b];
              $this->_children[$a] = $node_b;
              $this->_children[$b] = $node_a;
          }
      }
docs: Comparator method to rank child nodes by total weight.