XML Schema - only one element should have attribute set to true

Question

How can i define an boolean attribute that can be set "true" only in one element. Following snippet must be invalid.

<products>
  <product featured="yes">Prod 1</product>
  <product featured="yes">Prod 2</product>
</products>

Answer 1

You can't do that with XML Schemas.

You can define attributes on an element, but not limit them to one instance of the element.