Currently, output model mapping uses the following piece of XML Schema: 

<verbatim>
<xsd:element name="mapping">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element name="node" minOccurs="0" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:choice maxOccurs="unbounded">
            <xsd:element ref="concept"/>
            <xsd:element ref="literal"/>
            <xsd:element ref="variable"/>
          </xsd:choice>
          <xsd:attribute name="path" type="simpleXPathType" use="required"/>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
    <xsd:attribute name="automapping" type="xsd:boolean" use="optional" default="false"/>
  </xsd:complexType>
</xsd:element>
</verbatim>

The suggested changes would require this new schema:

<verbatim>
<xsd:element name="mapping">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element name="node" minOccurs="0" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:choice maxOccurs="unbounded">
            <xsd:element ref="interrelatableClass"/>
            <xsd:element ref="property"/>
            <xsd:element ref="literal"/>
            <xsd:element ref="variable"/>
          </xsd:choice>
          <xsd:attribute name="path" type="simpleXPathType" use="required"/>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>
</xsd:element>
</verbatim>

Where the new element references are defined as:

<verbatim>
<xsd:element name="property" type="propertyType" substitutionGroup="simpleExpression"/>

<xsd:complexType name="propertyType">
  <xsd:complexContent>
    <xsd:extension base="qualifiedConceptReferenceType">
      <xsd:attribute name="class" type="xsd:String" use="required"/>
    </xsd:extension>
  </xsd:complexContent>
</xsd:complexType>

<xsd:element name="class" type="classType"/>

<xsd:complexType name="classType">
  <xsd:complexContent>
    <xsd:extension base="qualifiedConceptReferenceType">
      <xsd:attribute name="alias" type="xsd:String" use="optional"/>
    </xsd:extension>
  </xsd:complexContent>
</xsd:complexType>

<xsd:element name="interrelatableClass" type="interrelatableClassType" ProposedChangesToOutputModelMapping>

<xsd:complexType name="interrelatableClassType">
  <xsd:complexContent>
    <xsd:extension base="classType">
       <xsd:sequence>
          <xsd:element ref="relationshipLink" maxOccurs="unbounded"/>
       </xsd:sequence>
    </xsd:extension>
  </xsd:complexContent>
</xsd:complexType>

<xsd:element name="relationshipLink" type="relationshipLinkType" ProposedChangesToOutputModelMapping>

<xsd:complexType name="relationshipLinkType">
  <xsd:complexContent>
    <xsd:extension base="qualifiedConceptReferenceType">
       <xsd:sequence>
          <xsd:element ref="interrelatableClass" minOccurs="1"/>
       </xsd:sequence>
      <xsd:attribute name="myRole" type="xsd:String" use="required"/>
      <xsd:attribute name="othersRole" type="xsd:String" use="required"/>
    </xsd:extension>
  </xsd:complexContent>
</xsd:complexType>

</verbatim>

Notes: 

   * The substitutionGroup attribute in the property element is related to its use in filters.
   * The class element is not directly used by the output model mapping, but it's a convenient definition that can be used by other parts of the protocol.