protected static function initMatrix(array $rules)
	{
		$matrix   = [];
		$tagNames = array_keys($rules);

		foreach ($rules as $tagName => $tagRules)
		{
			$matrix[$tagName]['allowedChildren']    = array_fill_keys($tagNames, 0);
			$matrix[$tagName]['allowedDescendants'] = array_fill_keys($tagNames, 0);
		}

		return $matrix;
	}