anchor.diff:      Board = new_game(5, 4, 4, [0,2,3,8]),
       {Events, #board{n_hidden = NHidden}} = step({2,2}, Board),
       ?assertEqual(6, NHidden),
  -    ?assertMatch([{uncover, 19, {empty, 0}},
  -                  {uncover, 16, {empty, 0}},
  -                  {uncover, 12, {empty, 1}},
  -                  {uncover, 17, {empty, 0}},
  -                  {uncover, 18, {empty, 0}},
  -                  {uncover, 15, {empty, 0}},
  -                  {uncover, 13, {empty, 1}},
  -                  {uncover, 14, {empty, 0}},
  -                  {uncover, 11, {empty, 0}},
  -                  {uncover,  9, {empty, 1}},
  -                  {uncover,  7, {empty, 2}},
  -                  {uncover,  6, {empty, 2}},
  -                  {uncover,  5, {empty, 3}},
  -                  {uncover, 10, {empty, 0}}], Events).
  +    ?assertMatch([{uncovered, 19, {empty, 0}},
  +                  {uncovered, 16, {empty, 0}},
  +                  {uncovered, 12, {empty, 1}},
  +                  {uncovered, 17, {empty, 0}},
  +                  {uncovered, 18, {empty, 0}},
  +                  {uncovered, 15, {empty, 0}},
  +                  {uncovered, 13, {empty, 1}},
  +                  {uncovered, 14, {empty, 0}},
  +                  {uncovered, 11, {empty, 0}},
  +                  {uncovered,  9, {empty, 1}},
  +                  {uncovered,  7, {empty, 2}},
  +                  {uncovered,  6, {empty, 2}},
  +                  {uncovered,  5, {empty, 3}},
  +                  {uncovered, 10, {empty, 0}}], Events).
   
   generate_mines_test() ->
       ?assertMatch([_,_,_], generate_mines(2*2, 3)),
  
positive.code:     %% mark end of game with game_over event
      Events1 = [game_over | UncoverEvents],
      {Events1, Board#board{n_hidden = 0}}.
  
  
  generate_mines(Fields, N) when N >= Fields -> throw(badarg);
  
negative.code:     Board = new_game(4, 4, 4, [0,2,3,8]),
      {Events, #board{n_hidden = NHidden}} = step({2,2}, Board),
      ?assertEqual(7, NHidden),
      ?assertMatch([{uncovered, 15, {empty, 0}},
                    {uncovered, 13, {empty, 1}},
                    {uncovered, 14, {empty, 0}},
                    {uncovered, 11, {empty, 0}},
                    {uncovered,  9, {empty, 1}},
                    {uncovered,  7, {empty, 2}},
                    {uncovered,  6, {empty, 2}},
                    {uncovered,  5, {empty, 3}},
                    {uncovered, 10, {empty, 0}}], Events).
  
  step_on_hole2_test() ->
      %%  *0, 1,*2,*3
  