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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_api_references(self, api_url=None):
 """Get set of HATEOAS reference for the given SCO-API. Use the default SCO-API if none is given. References are cached as they are not expected to change. Parameters Returns ------- """

        # Get subject listing Url for SCO-API
        if not api_url is None:
            url = api_url
        else:
            url = self.api_url
        # Check if API references are in local cache. If not send GET request
        # and add the result to the local cache
        if not url in self.apis:
            self.apis[url] = sco.references_to_dict(
                sco.JsonResource(url).json[sco.REF_LINKS]
            )
        return self.apis[url]