fun with a worm.  directionally, rather than positionally based

const qlength=20,unknown=0,north=1,east=2,south=3,west=4;

var qx{qlength},qy{qlength},tail,head,current,lastx,lasty,newx,newy;

procedure direction;
var x,y;
begin
  current:=unknown;
  if cvx>-1 then
    begin
      x:=cvx-250;
      y:=150-cvy;
      if -x<y then
        begin
          if x<y then
            current:=north;
          if x>y then
            current:=east
        end;
      if -x>y then
        begin
          if x>y then
            current:=south;
          if x<y then
            current:=west
        end
    end
end;

procedure enqueue(x,y);
  begin
    qx{tail}:=x;
    qy{tail}:=y;
    tail:=tail+1;
    if tail=qlength then
      tail:=0;
    if tail=head then
      begin
        head:=head+1;
        if head=qlength then
          head:=0
      end
  end;

procedure draw;
  var i;
  begin
    if tail#head then
      begin
        i:=head;
        while i#tail do
          begin
            call cvball(qx{i},qy{i},7);
            i:=i+1;
            if i=qlength then
              i:=0            
          end
      end
  end;

begin
  while 1=1 do
    begin
      call cvclear;
      while cvx<0 do
        call wait(10);
      tail:=0;
      head:=0;
      lastx:=250;
      lasty:=150;
      call enqueue(lastx,lasty);
      while cvx>-1 do
        begin
          call cvclear;
          call cvline(100,0,400,300);
          call cvline(100,300,400,0);
          call direction;
          if current#unknown then
            begin
              newx:=lastx;
              newy:=lasty;
              if current=north then
                newy:=lasty-10;
              if current=east then
                newx:=lastx+10;
              if current=south then
                newy:=lasty+10;
              if current=west then
                newx:=lastx-10;
              if newx>0 then
                if newx<500 then
                  if newy>0 then
                    if newy<300 then
                      begin
                        lastx:=newx;
                        lasty:=newy;
                        call enqueue(lastx,lasty)
                      end
            end;
          call draw;
          call wait(200)
        end
    end
end.