anchor.diff:  
   -spec to_string(proc_action()) -> string().
   
  -to_string({halt, Proc}) ->
  -    io_lib:format("Process ~s halts the system", [lid:to_string(Proc)]);
   to_string({block, Proc}) ->
       io_lib:format("Process ~s blocks", [lid:to_string(Proc)]);
   to_string({demonitor, Proc, not_found}) ->
  
positive.code:     io_lib:format("Process ~s unregisters process `~p`",
                    [lid:to_string(Proc), RegName]);
  to_string({whereis, Proc, RegName, not_found}) ->
      io_lib:format("Process ~s attempts to obtain the pid of unregistered "
  		  "process `~p` (undefined)",
                    [lid:to_string(Proc), RegName]);
  to_string({whereis, Proc, RegName, RegLid}) ->
      io_lib:format("Process ~s obtains the pid of process `~p` (~s)",
                    [lid:to_string(Proc), RegName, lid:to_string(RegLid)]).
  
negative.code: -type maybe_lid() :: lid:lid() | 'not_found'.
  
  %% Tuples providing information about a process' action.
  -type proc_action() :: {'block', lid:lid()} |
  		       {'demonitor', lid:lid(), maybe_lid()} |
                         {'exit', lid:lid(), term()} |
                         {'halt', lid:lid()} |
                         {'link', lid:lid(), maybe_lid()} |
  		       {'monitor', lid:lid(), maybe_lid()} |
  		       {'process_flag', lid:lid(), 'trap_exit', boolean()} |
  