anchor.diff:  -define(SERVER, ?MODULE).
   
   -record(state, {model :: model(),
  +                env :: environment(),
                   step = 1 :: pos_integer(),
                   log = false :: boolean(),
                   animate = true :: boolean()}).
  
positive.code: %%                                  {stop, Reason, State}
  %% @end
  %%--------------------------------------------------------------------
  handle_cast({log, _Env}, S = #state{log = false}) ->
      {noreply, S#state{step = S#state.step+1}};
  handle_cast({log, Env}, S) ->
      log(S#state.model, Env, S#state.step, S#state.animate),
      {noreply, S#state{step = S#state.step+1}};
  handle_cast(_Msg, S) ->
      {noreply, S}.
  
  %%--------------------------------------------------------------------
  %% @private
  
negative.code:     init([Model, Env, Log, true]);
  init([Model, Env, Log, Animate]) ->
      print(Model, Env, 1),
      {ok, #state{model = Model, log = Log, animate = Animate, env = Env}}.
  
  %%--------------------------------------------------------------------
  %% @private
  