Starts a new group for the given prefix.
        
        :param      prefix | <str>

def beginGroup(self, prefix):
        """
        Starts a new group for the given prefix.
        
        :param      prefix | <str>
        """
        curr = self._xstack[-1]
        next = curr.find(prefix)
        if next is None:
            next = ElementTree.SubElement(curr, prefix)
        self._xstack.append(next)