anchor.diff:  open System
   //open System.Diagnostics.Contracts
   open System.Globalization
  +open System.Runtime.InteropServices
   
   
   /// Represents a segment of a string.
  
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:     /// <returns></returns>
      [<CompiledName("Read")>]
      let read (substr : substring) : (char * substring) option =
          // 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 =
  