public function tag($tag, array $keys)
	{
		foreach ($keys as $key)
		{
			$resolvedKey = $this->resolveAlias($key);

			if (!isset($this->tags[$tag]))
			{
				$this->tags[$tag] = array();
			}

			$this->tags[$tag][] = $resolvedKey;
		}

		// Prune duplicates
		$this->tags[$tag] = array_unique($this->tags[$tag]);

		return $this;
	}