Scala Monad sequence method for generic travesables using for-comprehensions and builder pattern

MonadSequence.scala

import scala.language.higherKinds

def sequence[A, M[X] <: TraversableOnce[X]](in: M[Monad[A]])(implicit cbf: CanBuildFrom[M[Monad[A]], A, M[A]]): Monad[M[A]] = {
  val unit = Monad(cbf(in))
  in.foldLeft(unit){
    (mba, ma) => for (ba <- mba; a <- ma) yield ba += a
  }.map(_.result())
}