in mobile-save-for-later/src/main/scala/com/gu/sfl/lib/MergeLogic.scala [10:13]
def tailOrNil = if(l.isEmpty) Nil else l.tail
}
def mergeListBy[A, K](before: List[A], after: List[A])(key: A => K)(implicit ordering: Ordering[A]): List[A] = {