anchor.diff:  %%% EUnit
   %%%===================================================================
   
  --ifdef(TEST).
  +-spec local_compare(hashtree(), hashtree()) -> [keydiff()].
  +local_compare(T1, T2) ->
  +    Remote = fun(get_bucket, {L, B}) ->
  +                     get_bucket(L, B, T2);
  +                (key_hashes, Segment) ->
  +                     [{_, KeyHashes2}] = key_hashes(T2, Segment),
  +                     KeyHashes2
  +             end,
  +    compare(T1, Remote).
  +
  +-spec compare(hashtree(), remote_fun()) -> [keydiff()].
  +compare(Tree, Remote) ->
  +    compare(Tree, Remote, fun(Keys, KeyAcc) ->
  +                                  Keys ++ KeyAcc
  +                          end).
  +
  +-spec compare(hashtree(), remote_fun(), acc_fun(X)) -> X.
  +compare(Tree, Remote, AccFun) ->
  +    compare(Tree, Remote, AccFun, []).
   
   %% Verify that `update_tree/1' generates a snapshot of the underlying
   %% LevelDB store that is used by `compare', therefore isolating the
  
positive.code: -spec top_hash(hashtree()) -> [] | [{0, binary()}].
  top_hash(State) ->
      get_bucket(1, 0, State).
  
  -spec local_compare(hashtree(), hashtree()) -> [keydiff()].
  local_compare(T1, T2) ->
      Remote = fun(get_bucket, {L, B}) ->
                       get_bucket(L, B, T2);
                  (key_hashes, Segment) ->
                       [{_, KeyHashes2}] = key_hashes(T2, Segment),
                       KeyHashes2
               end,
      compare(T1, Remote).
  
  -spec compare(hashtree(), remote_fun()) -> [keydiff()].
  compare(Tree, Remote) ->
      compare(Tree, Remote, fun(Keys, KeyAcc) ->
                                    Keys ++ KeyAcc
                            end).
  
  -spec compare(hashtree(), remote_fun(), acc_fun(X)) -> X.
  compare(Tree, Remote, AccFun) ->
      compare(Tree, Remote, AccFun, []).
  
  compare(Tree, Remote, AccFun, Acc) ->
      compare(1, 0, Tree, Remote, AccFun, Acc).
  
negative.code:          destroy/1,
           read_meta/2,
           write_meta/3,
           compare/4,
           top_hash/1,
           get_bucket/3,
  