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:         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:         Returns a sequence containing every n-th element of the input sequence.
  
      *)
      
      
  
  
  /// Additional functional operators on immutable lists.
  