Possible Solution

Possible.scala

   trait SequenceValidation[B] {
    self =>
    def apply[A](seq: Seq[A]): DomainValidation[Seq[B]]

    def map[C](f: B => C) = new SequenceValidation[C] {
      def apply[A](seq: Seq[A]): DomainValidation[Seq[C]] =
        self.apply(seq) map { validSequence => validSequence map f }
    }

    def flatMap[C](f: B => SequenceValidation[C]) = new SequenceValidation[C] {
      def apply[A](seq: Seq[A]): DomainValidation[Seq[C]] = {
        val result: DomainValidation[Seq[DomainValidation[Seq[C]]]] = self.apply(seq) map  { validSequence =>
          validSequence map  { item =>
            f(item)(seq)
          }
        }
        result.flatMap { s =>
          s.sequence map { sp => sp.flatten}
        }
      }
    }
  }