anchor.diff:    }
     
     void blankCtx(html.CanvasRenderingContext2D ctx) {
  -    ctx.setFillColor(backgroundColor);
  +    ctx.fillStyle = backgroundColor;
       ctx.fillRect(0, 0, xmax, ymax);
     }
     
  
positive.code:   }
    
    void draw() {
      num baseHeading = TAU * (heading / 360);
      num deltaX = Math.cos(baseHeading);
      num deltaY = Math.sin(baseHeading);
      num newX = x + delta * deltaX;
  
negative.code:     cleanCtx(turtleCtx);
      if (delta != 0 && penupdown == PENDOWN) {
        userCtx.beginPath();
        userCtx.lineWidth = 2;
        userCtx.fillStyle = BLACK;
        userCtx.strokeStyle  = BLACK;
        userCtx.moveTo(x, y);
        userCtx.lineTo(newX, newY);
        userCtx.stroke();
  