public function single($alias)
	{
		$alias = strtolower($alias);

		// Return the existing object if it exists.
		if (! empty($this->instances[$alias]) && is_object($this->instances[$alias]))
		{
			return $this->instances[$alias];
		}

		// Die if we don't know what class to use.
		if (empty($this->services[$alias]))
		{
			throw new \InvalidArgumentException('Unable to find class with alias: '. $alias);
		}

		$instance = $this->make($alias, true);

		$this->instances[$alias] =& $instance;

		return $this->instances[$alias];
	}