anchor.diff:              |> List.collect (elemsDistance distance n a)
               |> if d <= n then List.cons b else id
   
  -    let fromList distance xs = xs |> List.fold (flip (insert distance)) empty
  +    let private fromList distance xs = xs |> List.fold (flip (insert distance)) empty
   
  -    let unions distance xs = xs |> List.collect elems |> fromList distance
  +    let private unions distance xs = xs |> List.collect elems |> fromList distance
   
  -    let rec delete distance a = function
  +    let rec private delete distance a = function
           | Empty -> Empty
           | Node(b, _, map) ->
               let d = distance a b
  
positive.code:         let (centerTree, _) = rightTree |> Map.splitWithKey ((>=) (d+n+1))
          centerTree
  
      let rec existsDistance distance n a = function
          | Empty -> false
          | Node(b, _, map) ->
              let d = distance a b
  
negative.code:             let dict = Map.insertWith recurse d (Node(a, 1, Map.empty)) dict
              Node(b, size + 1, dict)
  
      let rec private exists distance a = function
          | Empty -> false
          | Node(b, _, dict) ->
              let d = distance a b
  