protected function getPrefixes(array $tagNames)
	{
		$prefixes = [];
		foreach ($tagNames as $tagName)
		{
			$pos = strpos($tagName, ':');
			if ($pos !== false)
			{
				$prefixes[substr($tagName, 0, $pos)] = 1;
			}
		}
		$prefixes = array_keys($prefixes);
		sort($prefixes);

		return $prefixes;
	}