use this method to set the prefix of the QName,
        method looks in DOM to find prefix or set new prefix.
        This method must be called before get_formatted_content.

def set_prefix(self, elt, pyobj):
        '''use this method to set the prefix of the QName,
        method looks in DOM to find prefix or set new prefix.
        This method must be called before get_formatted_content.
        '''
        if isinstance(pyobj, tuple):
            namespaceURI,localName = pyobj
            self.prefix = elt.getPrefix(namespaceURI)