<document source="test data">
    <substitution_definition names="mydef">
        a substitution definition
    <paragraph>
        Now we can refer to 
        <substitution_reference refname="mydef">
            mydef
         using a substitution reference or to
        <substitution_reference ids="httpmy-url-com" names="http://my.url.com">
            mydef
         using a
        substitution-reference role.