anchor.diff:  agent_char(pos_z) -> $x;
   agent_char(neg_z) -> $*.
   
  --spec cell_char(cell()) -> char().
  +-spec cell_char(cell_state()) -> char().
   cell_char(dead) -> $.;
   cell_char(alive) -> $o.
  
positive.code: initial_cell_state() ->
      {dead}.
  
  -spec random_agent_state() -> direction().
  random_agent_state() ->
      [H | T] = ant_rule(),
      {lists:reverse(ant_rule()), H, T}.
  
negative.code: direction_to_heading(neg_y) -> {0, -1, 0};
  direction_to_heading(pos_z) -> {0, 0, 1};
  direction_to_heading(neg_z) -> {0, 0, -1}.
  
  %% displaying agents
  
  