Return a list of (uri,localname) tuples for all elements in the
        header that have mustUnderstand set.

def WhatMustIUnderstand(self):
        '''Return a list of (uri,localname) tuples for all elements in the
        header that have mustUnderstand set.
        '''
        return [ ( E.namespaceURI, E.localName )
                for E in self.header_elements if _find_mu(E) == "1" ]