anchor.diff:      if this.isKeyReferenceType && EqualityComparer<'K>.Default.Equals(key, Unchecked.defaultof<'K>) then raise (ArgumentNullException("key"))
       
       let mutable keepOrderVersion = false
  -    let entered = enterWriteLockIf &this.locker this.isSynchronized
  -    if entered then Interlocked.Increment(&this.nextVersion) |> ignore
  +    let mutable entered = false
       try
  +      try ()
  +      finally
  +        entered <- enterWriteLockIf &this.locker this.isSynchronized
  +        if entered then Interlocked.Increment(&this.nextVersion) |> ignore
         if this.size = 0 then
           this.Insert(0, key, value)
           keepOrderVersion <- true
  
positive.code:         member x.Clear() = raise (NotSupportedException("Values colelction is read-only"))
          member x.Contains(value) = this.ContainsValue(value)
          member x.CopyTo(array, arrayIndex) =
            let entered = enterWriteLockIf &this.locker this.IsSynchronized
            Array.Copy(this.values, 0, array, arrayIndex, this.size)
            exitWriteLockIf &this.locker entered
            
          member x.IndexOf(value:'V) = this.IndexOfValue(value)
          member x.Insert(index, value) = raise (NotSupportedException("Values collection is read-only"))
  
negative.code:         member x.Clear() = raise (NotSupportedException("Keys collection is read-only"))
          member x.Contains(key) = this.ContainsKey(key)
          member x.CopyTo(array, arrayIndex) =
            let mutable entered = false
            try
              entered <- enterWriteLockIf &this.locker this.IsSynchronized
              if couldHaveRegularKeys && this.size > 2 then
                Array.Copy(this.rkMaterialize(), 0, array, arrayIndex, this.size)
              else
                Array.Copy(this.keys, 0, array, arrayIndex, this.size)
            finally
              exitWriteLockIf &this.locker entered
  
          member x.IndexOf(key:'K) = this.IndexOfKey(key)
          member x.Insert(index, value) = raise (NotSupportedException("Keys collection is read-only"))
  