anchor.diff:  
   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   
  --spec get_XOR_fun() -> inner_hash_fun().
  -get_XOR_fun() ->
  -    fun([H|T]) -> lists:foldl(fun(X, Acc) -> X bxor Acc end, H, T) end.
  +-spec inner_hash_XOR(ChildHashes::[mt_node_key(),...]) -> mt_node_key().
  +inner_hash_XOR([H|T]) ->
  +    lists:foldl(fun(X, Acc) -> X bxor Acc end, H, T).
   
   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   
  
positive.code:          bucket_size    = 24                :: pos_integer(),   %max items in a leaf
           signature_size = 4                 :: pos_integer(),   %node signature size in byte
           leaf_hf        = fun(V) -> ?CRYPTO_SHA(V) end  :: hash_fun(),      %hash function for leaf signature creation
           inner_hf       = get_XOR_fun()     :: inner_hash_fun(),%hash function for inner node signature creation -
           keep_bucket    = false             :: boolean()        %false=bucket will be empty after bulk_build; true=bucket will be filled
           }).
  -type mt_config() :: #mt_config{}.
  
negative.code: 
  % allow future versions to create more hash funs by having an integer parameter
  -spec tester_create_inner_hash_fun(I::1) -> inner_hash_fun().
  tester_create_inner_hash_fun(1) -> fun inner_hash_XOR/1.
  