def _fold(header):
    """Fold a header line into multiple crlf-separated lines at column 72."""
    i = header.rfind(b'\r\n ')
    if i == -1:
        pre = b''
    else:
        i += 3
        pre = header[:i]
        header = header[i:]
    while len(header) > 72:
        i = header[:72].rfind(b' ')
        if i == -1:
            i = j = 72
        else:
            j = i + 1
        pre += header[:i] + b'\r\n '
        header = header[j:]
    return pre + header
