Pushes an (pos, effect) tuple onto a stack to later be executed if the
        state reaches the 'pos'.

def add_deferred_effect(self, effect, pos):
        """ Pushes an (pos, effect) tuple onto a stack to later be executed if the
        state reaches the 'pos'."""
        if not isinstance(pos, (unicode, str)):
            raise Exception("Invalid POS tag. Must be string not %d" % (type(pos)))
        if self['speaker_model']['is_syntax_stacked'] == True:
            self.__dict__['deferred_effects'].insert(0,(pos, effect,))
        elif self['speaker_model']['is_syntax_stacked'] == False:
            self.__dict__['deferred_effects'].append((pos, effect,))
        else:
            raise Contradiction("Speaker Model undefined")