public function parameter($key, $value = NULL)
	{
		if (func_num_args() === 1)
		{
			if ( ! array_key_exists($key, $this->_parameters))
			{
				throw new \InvalidArgumentException('Parameter not set: '.$key);
			}
			return $this->_parameters[$key];
		}
		$this->_parameters[$key] = $value;
		return $this;
	}