def does_contain_substr(stackstrings, new_ss):
    """
    Check if new string is a substring of an existing stackstring.
    :param stackstrings: list of all recovered stackstrings
    :param new_ss: new stackstring candidate
    :return: True if candidate substring of an existing stackstring, False otherwise
    """
    # IMPROVEMENT incorporate written_at in check
    for ss in filter(lambda es: es.fva == new_ss.fva, stackstrings):
        if new_ss.s in ss.s:
            return True
    return False