fetch all option instruments in an options chain
        - expiration_dates = optionally scope

def in_chain(cls, client, chain_id, expiration_dates=[]):
        """
        fetch all option instruments in an options chain
        - expiration_dates = optionally scope
        """

        request_url = "https://api.robinhood.com/options/instruments/"
        params = {
            "chain_id": chain_id,
            "expiration_dates": ",".join(expiration_dates)
        }

        data = client.get(request_url, params=params)
        results = data['results']

        while data['next']:
            data = client.get(data['next'])
            results.extend(data['results'])
        return results