code: protected function bound2() {
      if ( !is_null($this->_bound2) ) return $this->_bound2;
  
      $this->_bound2 = (($this->bound1() > $this->node->getRight()) ? $this->node->getRight() + 1 : $this->node->getLeft() - 1);
      return $this->_bound2;
    }
docs: Computes the other boundary.
  TODO: Maybe find a better name for this... ¿?
  
  @return int