anchor.diff:  
       let private Lock = obj ()
       let private Random = Random ()
  -    let mutable private Counter = -1L
       let private Cids = dictPlus<string, Guid> StringComparer.Ordinal []
       let private CidBytes = Array.zeroCreate 16
       let private CnameBytes = Array.zeroCreate 16
       let private UniqueAllocator = Unique.make ()
  +    let mutable private Counter = -1L
   
       /// Generates engine-specific values on-demand.
       type Gen =
  
positive.code:         static member free number =
              Unique.free number UniqueAllocator
  
  /// Generates engine-specific values on-demand.
  type Gen = Gen.Gen
negative.code:             | Some item -> item
              | None -> default_
  
          /// The prefix of a generated name
          static member namePrefix =
              "@"
  
          /// Generate a unique name.
          static member name =
              Gen.namePrefix + string Gen.id
  
          /// Generate a unique name if given none.
          static member nameIf nameOpt =
  