<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def url(self):
 """Returns the URL for the resource based on the ``self`` link. This method returns the ``href`` of the document's ``self`` link if it has one, or ``None`` if the document lacks a ``self`` link, or the ``href`` of the document's first ``self`` link if it has more than one. """

        if not 'self' in self.links:
            return None

        self_link = self.links['self']

        if isinstance(self_link, list):
            for link in self_link:
                return link.url()

        return self_link.url()