1  <grammar xmlns="http://relaxng.org/ns/structure/1.0">
 2    <start>
 3      <ref name="top"/>
 4    </start>
 5    <define name="empty">
 6      <empty/>
 7    </define>
 8    <define name="top">
 9      <element name="top">
10  	<ref name="empty"/>
11  	<element name="a">
12  	  <text/>
13  	</element>
14      </element>
15    </define>
16  </grammar>