optimistique sequance that prefer good result to bad ones

keepRight

 def keepRight[F[_], G[_, _], B, C](l: F[G[B, C]])(
    implicit //proofs
    G: Bifoldable[G],
    Me: scalaz.MonadError[G[B, ?], B],
    F: MonadPlus[F],
    Mobf: Monoid[F[B]], // proof we had a zero
    Moc: Monoid[F[C]], // proof we had a zero
    Mob: Monoid[B], // proof we can fold
    eqB: scalaz.Equal[F[B]],
    eqC: scalaz.Equal[F[C]],
    f: Foldable[F]
  ): G[B, F[C]] = {

    val (lefts, rights) = l.separate

    if (Mobf.isMZero(lefts) && Moc.isMZero(rights))
      Me.pure(Moc.zero)
    else if (Moc.isMZero(rights)) {
      Me.raiseError(lefts.fold)
    } else {
      Me.pure(rights)
    }

  }