Return a list of URI's of all the actor attributes found in
        the header.  The special actor "next" is ignored.

def WhatActorsArePresent(self):
        '''Return a list of URI's of all the actor attributes found in
        the header.  The special actor "next" is ignored.
        '''
        results = []
        for E in self.header_elements:
            a = _find_actor(E)
            if a not in [ None, SOAP.ACTOR_NEXT ]: results.append(a)
        return results