: fline ( x1 y1 x2 y2 --- )
  y2 ! x2 ! y1 ! x1 !
  flag off
  
  x2 @ x1 @ -		\ x2-x1
\  dup x0 !
  dup 0> if 1 else -1 then xdir !
  abs
  dup dx !		\ dx


  y2 @ y1 @ -		\ dx y2-y1
\  dup y0 !
  dup 0> if 1 else -1 then ydir !
  abs
  dup dy !		\ dx dy

  x1 @ y1 @ color @ plot

  < if			\
    x2 y2 exch  dx dy exch  x1 y1 exch  ( x0 y0 exch)  xdir ydir exch
    flag on
  then

  dy @ 2*		\ dy<<1
  dup dx @ - d !
  incr1 !		\
  dy @ dx @ - 2* incr2 !

\  x0 @ 0> if 1 else -1 then xdir !
\  y0 @ 0> if 1 else -1 then ydir !

  begin
    x1 @ x2 @ - while

    xdir @ x1 +!
    d @ 0< if
      incr1
    else
      ydir @ y1 +!
      incr2
    then
    @ d +!

    x1 @ y1 @ 
    flag @ if swap then
    color @ plot

  repeat
;


\ ========================================================================


: usefl
  ['] fline is line
;