anchor.diff:      UserId1 = <<"foo">>,
       UserId2 = <<"bar">>,
       UserId3 = <<"joe">>,
  +    UserId4 = <<"john">>,
       UserInfo1 = #{site => #{uid => UserId1}},
       UserInfo2 = #{site => #{uid => UserId2}},
       UserInfo3 = #{site => #{uid => UserId3}},
  +    UserInfo4 = #{site => #{uid => UserId4}},
  +
       CredState = <<"some_cred">>,
       Cred = <<"secret">>,
       MeckModules = [tts_data_sqlite, tts_cred_sup, tts_cred_worker, tts_service],
  
positive.code:                              UserId2 -> {ok, #{
                                             credential => [Cred],
                                             error => <<>>}, []};
                               _ -> {error, just_because, []}
                           end
                   end,
      ok = test_util:meck_new(MeckModules),
      ok = meck:expect(tts_data_sqlite, credential_add, AddFun),
      ok = meck:expect(tts_data_sqlite, credential_get_count, fun(_, _) -> {ok, 0} end),
  
negative.code:     Params = [],
      MyPid = self(),
      Interface = <<"test interface">>,
      AddFun = fun(User, Service, IFace, State, _AllowMultiple) ->
                       IFace = Interface,
                       User = UserId1,
                       Service = ServiceId,
  