Define a \(good~word\) as a sequence of letters that consists only of the letters \(A\), \(B\), and \(C\) - some of these letters may not appear in the sequence - and in which \(A\) is never immediately followed by \(B\), \(B\) is never immediately followed by \(C\), and \(C\) is never immediately followed by \(A\). How many seven-letter good words are there?

(第二十一届AIME2 2003 第3题)