anchor.diff:  
       (* TODO
   
  +    Seq.choosei
  +
       Seq.segment
           Groups elements of a sequence together "longitudinally" -- i.e., it works
           in a streaming fashion, rather than Seq.groupBy which needs to see the
  
positive.code:         Returns a sequence containing every n-th element of the input sequence.
  
      *)
  
  
  /// Additional functional operators on immutable lists.
  
negative.code: 
      (* TODO
  
      List.choosei
  
      List.unzipMap
          Similar to List.map2 (and List.zipMap, below). Given a list of tuples (a, b)
          applies the elements of each tuple to a function 'f' (f a b).
  