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