def angle(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      vec1Length = self.length ;
      vec2Length = rhs.length   ;
      return 0.0 if(vec1Length*vec2Length < self.tolerance )
      v = self.dot(rhs)/(vec1Length*vec2Length)
      Math::acos( v )
    end
def angle(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      vec1Length = self.length ;
      vec2Length = rhs.length   ;
      return 0.0 if(vec1Length*vec2Length < self.tolerance )
      v = self.dot(rhs)/(vec1Length*vec2Length)
      Math::acos( v )
    end
[Input]
  _rhs_ should be Vector3.
 [Output]
  return two vectors angle as Numeric (rad).