anchor.diff:  
       //
       [<CompiledName("IterBack")>]
  -    let iterBack action (substring : Substring) : unit =
  +    let iterBack action (substring : substring) : unit =
           let len = substring.Length
           for i = len - 1 downto 0 do
               action substring.[i]
   
       //
       [<CompiledName("Fold")>]
  -    let fold (folder : 'State -> char -> 'State) state (substring : Substring) : 'State =
  +    let fold (folder : 'State -> char -> 'State) state (substring : substring) : 'State =
           // OPTIMIZATION : Immediately return if the substring is empty.
           let len = substring.Length
           if len = 0 then state
  
positive.code:             notImpl "String.Split.iter"
  
          //
          let iteri (separator : char[]) (action : int -> string -> unit) (str : string) : unit =
              // Preconditions
              checkNonNull "separator" separator
              checkNonNull "str" str
  
negative.code:         // OPTIMIZE : The functions below could be modified to include an optimized case
          // for when the separator array contains just a single character.
  
          (*
          - String.Split.filter
          - String.Split.choose
            - These functions should work like .Split(...) |> Array.iter (or Array.fold, etc.),
              except that they won't actually need to traverse the entire string first and split
              it into an array of substrings. Instead, they'll improve performance by allowing us
              to execute a function on each of the substrings in a single forward pass.
            - Performance is improved with these functions because they'll be applied to Substring
              values instead of creating the substrings as separate objects.
            - This could be implemented by using the regular vector type / functions from fsharplex.
          *)
  
          //
          [<CompiledName("Iter")>]
          let iter (separator : char[]) (action : substring -> unit) (str : string) : unit =
              // Preconditions
              checkNonNull "separator" separator
              checkNonNull "str" str
  