public function add($snippet, Callable $callable=null) {
		if (is_object($snippet) && $snippet instanceof Snippets) {
			$this->addSnippetClass($snippet);
		}
		if (is_string($snippet) && is_callable($callable)) {
			// '$snippet' is a tag name: register $callable
			$this->snippets[$snippet] = $callable;
		}
	}