def _get_in_response_to(params):
    """
    Insert InResponseTo if we have a RequestID.
    Modifies the params dict.
    """
    request_id = params.get('REQUEST_ID', None)
    if request_id:
        params['IN_RESPONSE_TO'] = 'InResponseTo="%s" ' % request_id
    else:
        params['IN_RESPONSE_TO'] = ''
