defflatMap[B](f: (A) ⇒ GenTraversableOnce[B]): Seq[B][use case]Builds a new collection by applying a function to all elements of this sequence and using the elements of the resulting collections.
B the element typeof the returned collection.f the function to apply to each element.returns a new sequence resulting from applying the given collection-valued function f to each element of this sequence and concatenating the results.