def square[Element, IT[_] <: Iterable[Element], That](it: IT[Element])
          (implicit bf: CanBuildFrom[IT[Element], Element, That],
           num: Numeric[Element]): That = {
  val b = bf.apply(it)
  b.sizeHint(it)
  it.foreach(elem => b += num.times(elem,elem) )
  b.result()
}