@classmethod
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
