Bug in TREX tutorial 11.1? Lack of "combine"

Are the examples in the TREX tutorial section 11.1 incorrect in not using a
combine attribute when cell.content is defined in the embedded grammar after
a definition of cell.content has already be included?

	<grammar>
	  <include href="table.trex"/>      
	  <define name="cell.content">
	    <ref name="inline" parent="true"/>
	  </define>
	</grammar>
      
James