RE: optional, but at least one required

I have a similar situation that I'm having trouble with. I have a and b, at
least one of them has to appear once AND ONLY ONCE. a must come before b.
The examples in this thread allow more than one occurrence of a or b.

This is my first attempt at customizing a schema and I'm stuck.

TIA, Virginia

> You need to specify the requirements in a little more detail:
>
> * are multiple occurrences of a, b, and c allowed?
>
> * what constraints do you want to impose on the ordering of the elements?
>
> Michael Kay
> http://www.saxonica.com/