A sample XML schema describing a purchase order.
Business rules description:
a) If contents of billing-address are empty, then shipping-address must
have street1, city and country fields.
b) If contents of shipping-address are empty, then billing-address must
have street1, city and country fields.
c) If contents of both billing-address and shipping-address are non-empty,
then constraints for children of billing-address/shipping-address
specified in (a) and (b) need to be satisfied.
Tax is 10% of total order amount.
Total bill amount = 'Total order amount' + tax.