text: class M3dRotMatrix2d extends M3dMatrix2d {
    constructor() {
      super()
      this.angle = 0
      this.va = new M3dVector2d(Math.cos(this.alpha), Math.sin(this.alpha))
      this.vb = new M3dVector2d((-1) * Math.sin(this.alpha), Math.cos(this.alpha))
    }
  
    rotate(angle) {
      this.angle = angle
      this.va.set(Math.cos(angle), Math.sin(angle))
      this.vb.set((-1) * Math.sin(angle), Math.cos(angle))
    }
  
  }