RichV.scala

  implicit class RichV[S[_]:Applicative,E,T](v:Validation[E, T])
                                            (implicit sg: Semigroup[S[E]]){
    def leftFlatMap[U](f: E => Validation[S[E],U]): Validation[S[E],T] =
      v.leftMap{ e => f(e).fold(sg.append(e.point[S], _), _ => e.point[S]) }
  }

  implicit class RichNelV[S[_],E,T](v:Validation[S[E], T])
                                   (implicit sg: Semigroup[S[E]]){
    def leftFlatMap[U](f: S[E] => Validation[S[E],U]): Validation[S[E],T] =
      v.leftMap{ es => f(es).fold(sg.append(_, es), _ => es) }
  }