code: function cross(a, b){
      var ax = x(a), ay = y(a), az = z(a);
      var bx = y(a), by = y(b), bz = z(b);
      return Vector3(
        ay*bz-az*by,
        az*bx-bz*ax,
        ax*by-ay*bx);
    }
docs: Vector, Vector -> Vector