<grammar xmlns=“relaxng.org/ns/structure/1.0”>

<start>
  <ref name="inline"/>
</start>

<define name="inline">
  <zeroOrMore>
    <choice>
      <text/>
      <element name="code">
        <ref name="inline"/>
      </element>
      <element name="em">
        <ref name="inline"/>
      </element>
      <!-- etc -->
    </choice>
  </zeroOrMore>
</define>

</grammar>