anchor.diff:  %%--------------------------------------------------------------------
   %% @doc
   %% Updates the value of a key by executing a fun.
  +%%
  +%% This function updates a key/value pair by calling the provided
  +%% function and passing in the key and value as they are
  +%% currently. Additional arguments can be provided to the function by
  +%% passing them in the `Args' parameter. The entire operation is
  +%% executed within the scope of a new transaction.
   %%
   %% @spec update(Db, Key, Fun, Args) -> {ok, Value} | {error, Error}
   %% where
  
positive.code: 
  %%--------------------------------------------------------------------
  %% @doc
  %% Returns the current cursor value.
  %%
  %% @spec cursor_current() -> not_found | {ok, Key, Value} | {error, Error}
  %%
  
negative.code: 
  %%--------------------------------------------------------------------
  %% @doc
  %% Store a value in a database file and commit the transaction.
  %%
  %% This function is logically the equicalent of calling `put' followed
  %% by `txn_commit' except that it is implemented with the driver and is
  %% therefore more efficient.
  %%
  %% @spec put_commit(Db, Key, Value, Opts) -> ok | {error, Error}
  %% where
  