def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def serialize(self, value, display=False):
        if not value:
            return ""
        return "\n  " + "\n  ".join(encode(v) for v in value if v)