anchor.diff:      /// <returns></returns>
       [<CompiledName("Read")>]
       let read (substr : substring) : (char * substring) option =
  -        if substr.Length = 0 then None
  -        else
  -            // "Extract" the first (left-most) character from the substring.
  -            Some (substr.[0], substr.[1..])
  +        // Wrap the Substring.TryRead method in a more functional style.
  +        let mutable c = Unchecked.defaultof<_>
  +        let mutable remaining = Unchecked.defaultof<_>
  +        if substr.TryRead (&c, &remaining) then
  +            Some (c, remaining)
  +        else None
   
       //
       let inline private subUnsafe (substr : substring) offset count : substring =
  
positive.code:         if Seq.isEmpty source then
              System.String.Empty
          else
              let sb = System.Text.StringBuilder ()
              for substr in source do
                  sb.Append substr |> ignore
              sb.ToString ()
  
negative.code: open System
  //open System.Diagnostics.Contracts
  open System.Globalization
  open System.Runtime.InteropServices
  
  
  /// Represents a segment of a string.
  