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;
}
}