Retrieve citedby documents from a given PID number.

        pid: SciELO PID number
        metaonly: will retrieve only the metadata of the requested article citations including the number of citations it has received.
        from_heap: will retrieve the number of citations from a preproduced report, it will not fetch the api. Much faster results but not extremely updated.

def citedby_pid(self, pid, metaonly=False, from_heap=True):
        """
        Retrieve citedby documents from a given PID number.

        pid: SciELO PID number
        metaonly: will retrieve only the metadata of the requested article citations including the number of citations it has received.
        from_heap: will retrieve the number of citations from a preproduced report, it will not fetch the api. Much faster results but not extremely updated.
        """

        if from_heap is True:
            result = citations.raw_data(pid)

            if result and 'cited_by' in result and metaonly is True:
                del(result['cited_by'])
                return result

            if result:
                return result

        url = urljoin(self.CITEDBY_URL, self.PID_ENDPOINT)

        params = {
            "q": pid,
            "metaonly": "true" if metaonly is True else "false"
        }

        result = self._do_request(url, params=params)

        return result