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.