anchor.diff:  
   -export([
            get/0,
  +         get/1,
            check/2,
            create/1,
            update/1,
  
positive.code:              {stop, Reason :: term(), NewState :: #state{}}).
  handle_call({get}, _From, State) ->
      do_get_users(State);
  handle_call({create, User}, _From, State) ->
    do_create_user(State, User);
  handle_call({update, User}, _From, State) ->
  
negative.code: -spec(get() -> {ok, [Rec :: #user{}]} | {error, string()}).
  get() ->
      gen_server:call(?SERVER, {get}).
  
  -spec(get(Id :: integer()) -> {ok, [Rec :: #user{}]} | {error, string()}).
  get(Id) ->
    gen_server:call(?SERVER, {get, Id}).
  
  -spec(create(User :: #user{}) -> {ok, #user{}} | {error, string()}).
  create(User) ->
  