'Return a XML representation of the specified key, or None
if empty.  If _with_add_keys is False, dinamically generated
keys are excluded.'

def getAsXML(self, key, _with_add_keys=True):

    origModFunct = self.modFunct
    self.modFunct = modNull
    key = self.keys_alias.get(key, key)
    if ((not _with_add_keys) and (key in self._additional_keys())):
        self.modFunct = origModFunct
        return None
    try:
        withRefs = False
        if ((key in self.keys_tomodify) and (origModFunct not in (None, modNull))):
            withRefs = True
        value = self.get(key)
        if (value is None):
            return None
        tag = self.__class__.__name__.lower()
        return u''.join(_seq2xml({key: value}, withRefs=withRefs, modFunct=origModFunct, titlesRefs=self.titlesRefs, namesRefs=self.namesRefs, charactersRefs=self.charactersRefs, key2infoset=self.key2infoset, fullpath=tag))
    finally:
        self.modFunct = origModFunct
