Demo on well-formedness checking

<?xml version="1.0" encoding="UTF-8"?>
<people>
  <group>
    <person name="Al Zehtooney" age="33" sex="m" smoker="no"/>
    <person name="Brad York" age="38" sex="m" smoker="yes"/>
  </group>
  <group>
    <person name="Greg Sutter" age="40" sex="m" smoker="no"/>
    <person name="Harry Rogers" age="37" sex="m" smoker="no"/>
    <group>
      <person name="John Quincy" age="43" sex="m" smoker="yes"/>
      <person name="Kent Peterson" age="31" sex="m" smoker="no"/>
    </group>
    <person name="John Frank" age="24" sex="m" smoker="no"/>
  </group>
</people>

not-wf-1.xml:4: parser error : AttValue: " or ' expected
    <person name="Al Zehtooney" age=33 sex="m" smoker="no"/>
                                    ^
not-wf-1.xml:4: parser error : attributes construct error
    <person name="Al Zehtooney" age=33 sex="m" smoker="no"/>
                                    ^
not-wf-1.xml:4: parser error : Couldn't find end of Start Tag person line 4
    <person name="Al Zehtooney" age=33 sex="m" smoker="no"/>
                                    ^

not-wf-2.xml:6: parser error : Extra content at the end of the document
<group>
^

not-wf-3.xml:6: parser error : Opening and ending tag mismatch: person line 4 and group
  </group>
          ^
not-wf-3.xml:16: parser error : Opening and ending tag mismatch: group line 3 and people
</people>
         ^
not-wf-3.xml:17: parser error : Premature end of data in tag people line 2
^