code: function add_system_var(&$datatree, $index, $last, $key)
  	{
  		$datatree->_key = $key;
  		$datatree->_index = $index;
  		$datatree->_rank = $index + 1;
  		$datatree->_odd = $datatree->_not_even = (1 == $datatree->_rank % 2);
  		$datatree->_even = $datatree->_not_odd = (0 == $datatree->_rank % 2);
  		$datatree->_first = (0 == $index);
  		$datatree->_middle = !$datatree->_first && !$last;
  		$datatree->_last = $last;
  		$datatree->_not_first = !$datatree->_first;
  		$datatree->_not_last = !$last;
  		$datatree->_not_middle = !$datatree->_middle;
  	}
docs: Add system variables to an object