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<schema
 xmlns:a=".... a ....urn:a"
 targetNamespace="... a ...urn:a">
 
<element name="a_msg" type="a:a_msg_type"/>

... plus type definitions
</schema>

<schema
 xmlns:b="... b...urn:b"
 targetNamespace="... b ...urn:b">
 
<element name="b_msg" type="b:b_msg_type"/>

... plus type definitions
</schema>

<schema
 xmlns:c="... c...urn:c"
 targetNamespace="... c ...urn:c">
 
<element name="c_msg" type="c:c_msg_type"/>

... plus type definitions
</schema>

<schema
   targetNamespacexmlns="http://www.w3.org/2001/XMLSchema"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:a="urn:a"
  xmlns:b="urn:b"
  xmlns:c="urn:c"
  xmlns:abc ...="urn:abc"
  targetNamespace="urn:abc">

<import namespace="... a ...urn:a" schemaLocation="a.dfdl.xsd"/>
<import namespace="... b ...urn:b" schemaLocation="b.dfdl.xsd"/>
<import namespace="... c ...urn:c" schemaLocation="c.dfdl.xsd"/>

<element name="envelope" type="abc:envelopeType"/>

<complexType name="envelopeType">
  <sequence>
    <element name="typeCode" type="xs:unsignedInt"/>
    <choice dfdl:choiceDispatchKey='{ xs:string(typeCode) }'>
         <element ref="ma:a_msg" dfdl:choiceBranchKey="0" xmlns:m="...a..."/>
         <element ref="mb:b_msg" dfdl:choiceBranchKey="1" xmlns:m="...b..."/>
         <element ref="mc:c_msg" dfdl:choiceBranchKey="2" xmlns:m="...c..."/>
    </choice>
  </sequence>
</complexType>   

</schema>

<schema
   targetNamespacexmlns="http://www.w3.org/2001/XMLSchema"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:a="urn:a"
  xmlns:b="urn:b"
  xmlns:c="urn:c"
  xmlns:abc ...="urn:abc"
  targetNamespace="urn:abc">

<import namespace="... a ...urn:a" schemaLocation="a.dfdl.xsd"/>
<import namespace="... b ...urn:b" schemaLocation="b.dfdl.xsd"/>
<import namespace="... c ...urn:c" schemaLocation="c.dfdl.xsd"/>

<element name="envelope" type="abc:envelopeType"/>

<complexType name="envelopeType">
  <sequence>

      <sequence <sequence dfdl:hiddenGroupRef="abc:h_typeCode"/>   <!-- DIFFERENT: Use a HIDDEN GROUP. -->       

      <choice dfdl:choiceDispatchKey='{ xs:string(typeCode) }'>
          <element ref="ma:a_msg" dfdl:choiceBranchKey="0" xmlns:m="...a..."/>
          <element ref="mb:b_msg" dfdl:choiceBranchKey="1" xmlns:m="...b..."/>
          <element ref="mc:c_msg" dfdl:choiceBranchKey="2" xmlns:m="...c..."/>
    </choice>
  </sequence>
</complexType>   

<!--
Up to this point, only one line of the schema was different. The one that
references the hidden group below. 

This hidden group at parse time will populate the typeCode element.

The trick to keep it from unparsing is this:
At unparse time the elements in the hidden group are not part of the
infoset, so when unparsing, the unparser will have nothing
to go on when it encounters the need to unparse this hidden choice group.
Now, dfdl:choiceDispatchKey is only evaluated when parsing, not
when unparsing so the unparser has nothing to go on, so will choose the 
first branch of the choice here, which contains nothing so nothing gets unparsed. 
-->

<group name="h_typeCode">
  <choice dfdl:choiceDispatchKey='{ "parse" }'>
     <sequence dfdl:choiceBranchKey="unparse">
       <!-- Don't unparse anything. 
            By default, when unparsing a choice, 
            the first branch is taken -->
     </sequence>
     <sequence dfdl:choiceBranchKey="parse">
       <element name="typeCode" type="xs:unsignedInt"
          dfdl:outputValueCalc='{ 0 }'>
         <!--
         And there is this one additional annoying detail:

	     The dfdl:outputValueCalc above is only present to satisfy the
         Daffodil schema compiler. All elements in hidden groups must have
         a way to unparse, which means they are defaultable or have
         a dfdl:outputValueCalc, or it will not schema-compile.
         Daffodil's compiler is not quite smart enough to realize this 
         element will never be unparsed so the schema compiler insists on having 
         a dfdl:outputValueCalc even though it the property will never be used.
         --> 
       </element>
     </sequence>
  </choice>
</group>


</schema>