code: public Quaterniond add(double x, double y, double z, double w) {
          return add(x, y, z, w, this);
      }
docs: Add the quaternion <code>(x, y, z, w)</code> to this quaternion.
  
  @param x
  the x component of the vector part
  @param y
  the y component of the vector part
  @param z
  the z component of the vector part
  @param w
  the real/scalar component
  @return this