Validate specific XML with XSD

We have specific XML that has specific structure:

<root>
  <element type=”a”>
    <value>someValueA</value>
  </element>
  <element type=”b”>
    <value>someValueB</value>
  </element>
</root>

Is it possible to validate it usi开发者_开发百科ng XSD in such a way that the element with type “a” is required and that of type “b” is not required?

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In XSD 1.0 you can't apply different constraints to different sibling elements that have the same element name.

It looks like you need XSD 1.1 assertions - available now in Saxon and Xerces.