I have attached PDF from my book about using a string variable as a bitmap. It is similar to using a counter as a bitmap, but it works for a different DTD situation (a sequence of three elements, each of which is optional). I came up with this for some quick-and-dirty proof-of-concept code that was never completed. This approach seemed to be the easiest way to handle all the possibilities.
Please let me know any comments, questions, corrections, etc.