docs: 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