<document source="test data">
    <paragraph>
        <problematic ids="id2" refid="id1">
            |
        This is not
        <problematic ids="id4" refid="id3">
            |
        a line block
    <system_message backrefs="id2" ids="id1" level="2" line="1" source="test data" type="WARNING">
        <paragraph>
            Inline substitution_reference start-string without end-string.
    <system_message backrefs="id4" ids="id3" level="2" line="2" source="test data" type="WARNING">
        <paragraph>
            Inline substitution_reference start-string without end-string.
    <line_block>
        <line>
            This is an
    <system_message level="2" line="5" source="test data" type="WARNING">
        <paragraph>
            Line block ends without a blank line.
    <paragraph>
        <problematic ids="id6" refid="id5">
            |
        incomplete line block.
    <system_message backrefs="id6" ids="id5" level="2" line="5" source="test data" type="WARNING">
        <paragraph>
            Inline substitution_reference start-string without end-string.