anchor.diff:          let (centerTree, _) = rightTree |> Map.splitWithKey ((>=) (d+n+1))
           centerTree
   
  -    let rec existsDistance distance n a = function
  +    let rec private existsDistance distance n a = function
           | Empty -> false
           | Node(b, _, map) ->
               let d = distance a b
  
positive.code: 
      let singleton a = Node(a, 1, Map.empty)
  
      let rec insert distance a = function
          | Empty -> Node(a, 1 , Map.empty)
          | Node(b, size, dict) ->
              let recurse _ tree = insert distance a tree
  
negative.code:         | Node(a, _ , d) ->
              a :: (d |> Map.valueList |> List.collect elems)
  
      let rec private elemsDistance distance n a = function
          | Empty -> []
          | Node(b, _, dict) ->
              let d = distance a b
  