'Make the given L{Node} the I{document element} of this L{Document}.
@param child: The L{Node} to make into this L{Document}\'s document
element.
@raise ValueError: If this document already has a document element.'

def appendChild(self, child):

    if self.childNodes:
        raise ValueError('Only    one    element    per    document.')
    Node.appendChild(self, child)
