anchor.diff:          interface INode with
   
               member this.assoc(shift, hashKey, key, value, addedLeaf) : INode = 
  -                let idx = NodeHelpers.mask(hashKey, shift)
  +                let idx = BitCount.mask(hashKey, shift)
                   let node = this.array.[idx]
                   if node = Unchecked.defaultof<INode> then
                       ArrayNode(ref null, this.count + 1, NodeHelpers.cloneAndSet(this.array, idx, (BitmapIndexedNode() :> INode).assoc(shift + 5, hashKey, key, value, addedLeaf))) :> INode
  
positive.code:                     null
  
              member this.tryFind(shift, hash, key) =
                  let bit = NodeHelpers.bitpos(hash, shift)
                  if this.bitmap &&& bit = 0 then None else
                  let idx = NodeHelpers.index(this.bitmap,bit)
                  let keyOrNull = this.array.[2*idx]
                  let valOrNode = this.array.[2*idx+1]
                  if keyOrNull = null then (valOrNode :?> INode).tryFind(shift + 5, hash, key) else
  
negative.code: 
  
              member this.assoc(thread1, shift, hashKey, key, value, addedLeaf) = 
                  let bit = BitCount.bitpos(hashKey, shift)
                  let idx = BitCount.index(this.bitmap,bit)
                  if (this.bitmap &&& bit) <> 0 then
                      let keyOrNull = this.array.[2*idx]
                      let valOrNode = this.array.[2*idx+1]
  