: XYPOS [ GRID_MEM LITN ] ;
: AT-XY ( x y -- ) [ ROWS LITN ] * +
    [ COLS ROWS * LITN ] MOD XYPOS ! ;
: _lf
    XYPOS @ BEGIN ( pos )
        0 ( blank ) SWAP TUCK CELL!
        1+ DUP [ COLS LITN ] MOD NOT UNTIL
    [ COLS ROWS * LITN ] MOD XYPOS ! ;
: _bs 0 ( blank ) XYPOS @ TUCK CELL! ( pos ) 1-
    [ COLS ROWS * LITN ] MOD XYPOS ! ;