program test(input, output);

uses gralib;

label 99;

const accel = 20; 

var x, y, xd, yd, i: integer;

procedure wait;

var er: evtrec; { event record }

begin

   repeat event(input, er) until (er.etype = etframe) or (er.etype = etterm);
   if er.etype = etterm then goto 99

end;

begin

   auto(output, false);
   curvis(output, false);
   fcolor(output, white);
   fxor(output);
   x := 1;
   y := 1;
   xd := +1;
   yd := +1;
   frame(output, true);
   while true do begin

      for i := 1 to accel do begin

         setpixel(output, x, y);
         x := x+xd;
         y := y+yd;
         if (x = 1) or (x = maxxg(output)) then xd := -xd;
         if (y = 1) or (y = maxyg(output)) then yd := -yd

      end;
      wait
 
   end;

   99:

   auto(output, true);
   curvis(output, true)

end.