anchor.diff:    }
     
     void blankCtx(html.CanvasRenderingContext2D ctx) {
  -    ctx.setFillColor(backgroundColor);
  +    ctx.fillStyle = backgroundColor;
       ctx.fillRect(0, 0, xmax, ymax);
     }
     
  
positive.code:     cleanCtx(turtleCtx);
      if (delta != 0 && penupdown == PENDOWN) {
        userCtx.beginPath();
        userCtx.setLineWidth(2);
        userCtx.setFillColor(BLACK);
        userCtx.setStrokeColor(BLACK);
        userCtx.moveTo(x, y);
        userCtx.lineTo(newX, newY);
        userCtx.stroke();
  
negative.code:   
    void drawFillCircle(ctx, num x_, num y_, num radius, color) {
      ctx.beginPath();
      ctx.lineWidth = 2;
      ctx.strokeStyle = color;
      ctx.fillStyle = color;
      ctx.arc(x_, y_, radius, 0, TAU, false);
      ctx.fill();
    }
  