anchor.diff:  
   /// Try convert the keys of a specified zoo time series to DateTime
   let tryGetDateTimeKeys (zoo:SymbolicExpression) fromDateTime =
  -  let invcult = System.Globalization.CultureInfo.InvariantCulture
     try
       R.strftime(R.index(zoo), "%Y-%m-%d %H:%M:%S").AsCharacter()
       |> Seq.map (fun v -> DateTime.ParseExact(v, "yyyy-MM-dd HH:mm:ss", invcult))
  
positive.code: // Conversion helpers
  // ------------------------------------------------------------------------------------------------
  
  /// Convert Deedle frame key (or multi-level tuple) to string that can be passed to R
  let convertKey (key:obj) =
    if Object.ReferenceEquals(key, null) then ""
  
negative.code:     FSharpValue.GetTupleFields(key)
      |> Array.map string
      |> String.concat " - "
    elif key :? DateTime then
      dateTimeToStr (key :?> DateTime)
    elif key :? DateTimeOffset then
      dateTimeOffsetToStr (key :?> DateTimeOffset)
    else string key
  
  /// Turn columns/rows into an index with either int or string keys
  