static int prune_dcs(t30_state_t *s)
{
    int i;

    /* Find the last octet that is really needed, set the extension bits, and trim the message length */
    for (i = 18;  i > 4;  i--)
    {
        /* Strip the top bit */
        s->dcs_frame[i] &= (DISBIT1 | DISBIT2 | DISBIT3 | DISBIT4 | DISBIT5 | DISBIT6 | DISBIT7);
        /* Check if there is some real message content here */
        if (s->dcs_frame[i])
            break;
    }
    s->dcs_len = i + 1;
    /* Fill in any required extension bits */
    for (i--  ;  i > 4;  i--)
        s->dcs_frame[i] |= DISBIT8;
#if 1
    s->dcs_frame[0] = 0xFF;
    s->dcs_frame[1] = 0x13;
    s->dcs_frame[2] = 0x83;
    s->dcs_frame[3] = 0x00;
    s->dcs_frame[4] = 0xC6;
    s->dcs_frame[5] = 0x98;
    s->dcs_frame[6] = 0x00;
    s->dcs_len = 7;
#endif
    t30_decode_dis_dtc_dcs(s, s->dcs_frame, s->dcs_len);
    return s->dcs_len;
}