Schema issue: Order dependence of submission child elements

The issue of ordered children of submission came up on the Wednesday 
telecon.

On the telecon it was stated that it was hard in XML schema to specify 
child elements that were both optional and in a required order.

I preserved this aspect of submission in the current editor's draft 
because I do not understand why it is hard, whereas I can easily see it 
being harder to write a schema that says the child is optional but could 
appear anywhere in the list of child elements.  It is easy to see how to 
do it if the schema also allows the child to appear any number of times, 
but this is not what we want.

Isn't it just a case of saying that the submission has a resource element 
with minoccurs=0 then a method element with minoccurs=0 then a header 
element with minoccurs=0 and maxoccurs=unbounded?

Is there an easy way to say "In any order, one or zero resource plus one 
or zero method plus zero or more header"? 

Cheers,
John M. Boyer, Ph.D.
STSM: Lotus Forms Architect and Researcher
Chair, W3C Forms Working Group
Workplace, Portal and Collaboration Software
IBM Victoria Software Lab
E-Mail: boyerj@ca.ibm.com 

Blog: http://www.ibm.com/developerworks/blogs/page/JohnBoyer