lib/asciidoctor/iso/basicdoc.rng in metanorma-iso-1.6.1 vs lib/asciidoctor/iso/basicdoc.rng in metanorma-iso-1.7.0

@@ -727,12 +727,14 @@
       <empty/>
     </element>
   </define>
   <define name="index">
     <element name="index">
-      <attribute name="to">
-        <data type="IDREF"/>
-      </attribute>
+      <optional>
+        <attribute name="to">
+          <data type="IDREF"/>
+        </attribute>
+      </optional>
       <element name="primary">
         <oneOrMore>
           <ref name="PureTextElement"/>
         </oneOrMore>
       </element>