XSD requires an id element before the first field element

The entity element in doctrine-mapping.xsd requires an element id in front of the first element field.

This leads to invalid XML files, where a inherited class doesn't need an ID on its own, but inherits one from its parent.

In the examples in the documentation,
http://www.doctrine-project.org/projects/orm/2.0/docs/reference/xml-mapping/en#xml-element-reference:defining-fields
the code, too, has no id element in front of the first field elements.

A minimal example, showing the inheritance problem, too, is attached. The validation error occurs on the field element of the "Cat" entity.