anchor.diff:  
     override this.TryGetValue(key:'K, isMove:bool, [<Out>] value: byref<'R>): bool =
       if isMove then
  -      if laggedCursor = Unchecked.defaultof<_> then laggedCursor <- this.InputCursor.Clone()
  +      if laggedCursor = Unchecked.defaultof<_> then 
  +        laggedCursor <- this.InputCursor.Clone()
  +      else
  +        let moved = laggedCursor.MoveAt(key, Lookup.EQ)
  +        if not moved then raise (ApplicationException("This should not happen by design"))
         #if PRERELEASE
         Trace.Assert(laggedCursor.Comparer.Compare(laggedCursor.CurrentKey,key) = 0 )
         Trace.Assert(laggedCursor.Comparer.Compare(this.InputCursor.CurrentKey,key) = 0 )
         #endif
  -//      let moved = laggedCursor.MoveAt(key, Lookup.EQ)
  -//      if not moved then raise (ApplicationException("This should not happen by design"))
         currentLag <- 0u
         let mutable cont = true
         while currentLag < zeroBasedLag && cont do
  
positive.code: 
    override this.TryGetValue(key:'K, isMove:bool, [<Out>] value: byref<'R>): bool =
      if isMove then
        if laggedCursor = Unchecked.defaultof<_> then laggedCursor <- this.InputCursor.Clone()
        #if PRERELEASE
        Trace.Assert(laggedCursor.Comparer.Compare(laggedCursor.CurrentKey,key) = 0 )
        Trace.Assert(laggedCursor.Comparer.Compare(this.InputCursor.CurrentKey,key) = 0 )
        #endif
  //      let moved = laggedCursor.MoveAt(key, Lookup.EQ)
  //      if not moved then raise (ApplicationException("This should not happen by design"))
        currentWidth <- 0u
        currentState <- initState.Invoke()
  
  
negative.code:   
    override this.TryGetValue(key:'K, isMove:bool, [<Out>] value: byref<'R>): bool =
      if isMove then
        if laggedCursor = Unchecked.defaultof<_> then 
          laggedCursor <- this.InputCursor.Clone()
        else 
          let moved = laggedCursor.MoveAt(key, Lookup.EQ)
          if not moved then raise (ApplicationException("This should not happen by design"))
        #if PRERELEASE
        Trace.Assert(laggedCursor.Comparer.Compare(laggedCursor.CurrentKey,key) = 0 )
        Trace.Assert(laggedCursor.Comparer.Compare(this.InputCursor.CurrentKey,key) = 0 )
        #endif
        currentLag <- 0u
        let mutable cont = true
        while currentLag < lag && cont do
  