code: public void add(Vector3d v1, Vector3d v2) {
          x = v1.x + v2.x;
          y = v1.y + v2.y;
          z = v1.z + v2.z;
      }
docs: Adds vector v1 to v2 and places the result in this vector.
  
  @param v1
  left-hand vector
  @param v2
  right-hand vector