code: function Mat33(a, b, c) {
    if (typeof a === 'object' && a !== null) {
      this.ex = Vec3.clone(a);
      this.ey = Vec3.clone(b);
      this.ez = Vec3.clone(c);
    } else {
      this.ex = Vec3();
      this.ey = Vec3();
      this.ez = Vec3();
    }
  }
docs: A 3-by-3 matrix. Stored in column-major order.