BFOVERWRITE
if (curbp->b_flag & BFOVERWRITE) {
curbp->b_flag &= ~BFOVERWRITE;
curbp->b_flag |= BFOVERWRITE;
curbp->b_flag ^= BFOVERWRITE;
defb_flag &= ~BFOVERWRITE;
defb_flag |= BFOVERWRITE;