<document source="test data">
    <substitution_definition names="mydef">
        a substitution definition
    <paragraph>
        Now we can refer to 
        a substitution definition
         using a substitution reference or to
        a substitution definition
         using a substitution-reference role or
        to 
        <inline classes="custom">
            a substitution definition
         using a custom role based off
        substitution-reference.