def copy(self):
        """Return a copy of this `Fact`."""
        content = [(k, v) for k, v in self.items()]

        intidx = [(k, v) for k, v in content if isinstance(k, int)]
        args = [v for k, v in sorted(intidx)]

        kwargs = {k: v
                  for k, v in content
                  if not isinstance(k, int) and not self.is_special(k)}
        return self.__class__(*args, **kwargs)
Return a copy of this `Fact`.