Void Attributes (∅) Not Higher Than Other

In XMIR, all void attributes must be placed on top of other non-void attributes.

Incorrect:

<o name="foo">
  <o base="∅" name="x"/>
  <o base="Q.foo" name="z"/>
  <o base="∅" name="y"/>
</o>

Correct:

<o name="foo">
  <o base="∅" name="x"/>
  <o base="∅" name="y"/>
  <o base="Q.foo" name="z"/>
</o>