There is currently no way to do anything backward except than setting the position of the pointer to a known position (that is, before the current pointer position).
I'd like to see this added:
1) /fseek -p, similar to -n, would set the pointer to the beginning of the previous line (-n is probably just a loop on $fgetc until $lf is seen forward, which is what I currently do, but backward which is not efficient in msl).
2) $fopen().line, which would return the line number the pointer is on
3) $fread().back, read the next line or N bytes from the pointer, but backward