diff --git a/xml/constraints.rng b/xml/constraints.rng
index 42ab90c690..4ce74ebd6c 100644
--- a/xml/constraints.rng
+++ b/xml/constraints.rng
@@ -1,110 +1,112 @@
 <?xml version="1.0" encoding="utf-8"?>
 <grammar xmlns="http://relaxng.org/ns/structure/1.0" 
          datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
   <start>
       <ref name="element-constraints"/>
   </start>
 
   <define name="element-constraints">
       <zeroOrMore>
 	<choice>
 	  <ref name="element-location"/>
 	  <ref name="element-colocation"/>
 	  <ref name="element-order"/>
 	</choice>
       </zeroOrMore>
   </define>
 
   <define name="element-location">
     <element name="rsc_location">
       <attribute name="id"><data type="ID"/></attribute>
       <attribute name="rsc"><data type="IDREF"/></attribute>
       <choice>
 	<group>
 	  <externalRef href="score.rng"/>
 	  <attribute name="node"><data type="IDREF"/></attribute>
 	</group>
 	<oneOrMore>
 	  <externalRef href="rule.rng"/>
 	</oneOrMore>
       </choice>
       <optional>
 	<ref name="element-lifetime"/>
       </optional>
     </element>
   </define>
 
   <define name="element-colocation">
     <element name="rsc_colocation">
       <attribute name="id"><data type="ID"/></attribute>
       <attribute name="from"><data type="IDREF"/></attribute>
       <attribute name="to"><data type="IDREF"/></attribute>
       <externalRef href="score.rng"/>
       <optional>
 	<ref name="element-lifetime"/>
       </optional>
       <optional>
 	<attribute name="from_role">
 	  <ref name="attribute-roles"/>
 	</attribute>
       </optional>
       <optional>
 	<attribute name="to_role">
 	  <ref name="attribute-roles"/>
 	</attribute>
       </optional>
     </element>
   </define>
 
   <define name="element-order">
     <element name="rsc_order">
       <attribute name="id"><data type="ID"/></attribute>
       <attribute name="from"><data type="IDREF"/></attribute>
       <attribute name="to"><data type="IDREF"/></attribute>
-      <externalRef href="score.rng"/>
+      <optional>
+	<externalRef href="score.rng"/>
+      </optional>
       <optional>
 	<ref name="element-lifetime"/>
       </optional>
       <optional>
 	<attribute name="symmetrical"><data type="boolean"/></attribute>
       </optional>
       <optional>
 	<attribute name="action">
 	  <ref name="attribute-actions"/>
 	</attribute>
       </optional>
       <optional>
 	<attribute name="to_action">
 	  <ref name="attribute-actions"/>
 	</attribute>
       </optional>
     </element>
   </define>
  
   <define name="attribute-actions">
     <choice>
       <value>start</value>
       <value>promote</value>
       <value>demote</value>
       <value>stop</value>
     </choice>
   </define>
       
   <define name="attribute-roles">
     <choice>
       <value>Stopped</value>
       <value>Started</value>
       <value>Master</value>
       <value>Slave</value>
     </choice>
   </define>
 
   <define name="element-lifetime">
     <element name="lifetime">
       <oneOrMore>
 	<externalRef href="rule.rng"/>
       </oneOrMore>
     </element>
   </define>
   
 </grammar>