def _seq_of_Ns_from_db(dotbracket):
    """
    Get a sequence containing only 'N'-characters with the same length and
    the same position of cutpoints as the dotbracketstring.

    :param dotbracket: A string, optionally containing '&' to indicate
                       seperate cofolded structures.
    """
    seq = []
    for substr in dotbracket.split('&'):
        seq.append('N' * len(substr))
    return '&'.join(seq)
