`substitution_reference`` will simply be replaced by the
contents of the corresponding ``substitution_definition``.

The transformed result will be::

    <document>
        <paragraph>
            The
            <image alt="biohazard" uri="biohazard.png">
             symbol is deservedly scary-looking.
        <substitution_definition name="biohazard">
            <image alt="biohazard" uri="biohazard.png">
ιά