l.x + 50, this.prevmark.l.y + 50);
    this.engine.ctx.lineTo(l.x + 50, l.y + 50);

    this.engine.ctx.moveTo(this.prevmark.r.x + 50, this.prevmark.r.y + 50);
    this.engine.ctx.lineTo(r.x + 50, r.y + 50);
    this.engine.ctx.stroke();
  }

  this.prevmark = {
    l: l,
    r: r
  }
}
            </code></pre>
          </section>
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