public function set_attribute($attr, $value = null)
	{
		$attr = is_array($attr) ? $attr : array($attr => $value);
		foreach ($attr as $key => $value)
		{
			if ($value === null)
			{
				unset($this->attributes[$key]);
			}
			else
			{
				$this->attributes[$key] = $value;
			}
		}

		return $this;
	}