anchor.diff:          Returns a sequence containing every n-th element of the input sequence.
   
       *)
  +    
  +    
   
   
   /// Additional functional operators on immutable lists.
  
positive.code:         state
  
      // TODO :
      // foldPairs, foldBackPairs
      // derive    // takes a 'T -> 'T -> 'T like reduce, but only performs one step; used to perform 'divided differences'
          // Other possible names: reduceOnce, reduceStep
  
negative.code: 
      (* 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
  