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      <choice>
11        <ref name="empty"/>
12        <element name="a">
13	  <text/>
14        </element>
15      </choice>
16    </element>
17  </define>
18</grammar>
19