anchor.diff:    {stop, Reason :: term(), NewState :: #state{}}).
   handle_call({get}, _From, State) ->
     do_get_devices(State);
  -handle_call({create, User}, _From, State) ->
  -  do_create_device(State, User);
  +handle_call({create, Device}, _From, State) ->
  +  do_create_device(State, Device);
  +handle_call({update, Device}, _From, State) ->
  +  do_update_device(State, Device);
   handle_call(_Request, _From, State) ->
     {reply, ok, State}.
   
  
positive.code: -spec(create(Device :: #device{}) -> {ok, #device{}} | {error, string()}).
  create(Device) ->
    gen_server:call(?SERVER, {create, Device}).
  
  %%--------------------------------------------------------------------
  %% @doc
  
negative.code: 
  -export([
    get/0,
    create/1,
    update/1
  ]).
  
  %% gen_server callbacks
  