anchor.diff:      true;
   node_postcondition(_NodeState, {call, ?MODULE, broadcast, [_Node, _Message]}, ok) ->
       true;
  -node_postcondition(#node_state{sent=Sent}, {call, ?MODULE, check_mailbox, []}, Results) ->
  -    lists:foldl(fun({Message, Recipients}, MessageAcc) ->
  +node_postcondition(#node_state{failed_to_send=FailedToSend, sent=Sent}, {call, ?MODULE, check_mailbox, []}, Results) ->
  +    AllSentAndReceived = lists:foldl(fun({Message, Recipients}, MessageAcc) ->
           node_debug("verifying that message ~p was received by ~p", [Message, Recipients]),
   
           All = lists:foldl(fun(Recipient, NodeAcc) ->
  
positive.code: %% What should the initial node state be.
  node_initial_state() ->
      node_debug("initializing", []),
      #node_state{sent=[]}.
  
  %% Names of the node functions so we kow when we can dispatch to the node
  %% pre- and postconditions.
  
negative.code:         end, true, Recipients),
  
          All andalso MessageAcc
      end, true, Sent),
  
      FailedToSendNotReceived = lists:foldl(fun({Message, Recipients}, MessageAcc) ->
          node_debug("verifying that message ~p was NOT received by ~p", [Message, Recipients]),
  
          All = lists:foldl(fun(Recipient, NodeAcc) ->
              node_debug("=> verifying that message ~p was NOT received by ~p", [Message, Recipient]),
  
              %% Did this node receive the message?
              Messages = dict:fetch(Recipient, Results),
  
              %% Carry forward.
              case lists:member(Message, Messages) of 
                  true ->
                      node_debug("=> => message received at node ~p: ~p", [Message, Recipient]),
                      NodeAcc andalso false;
                  false ->
                      NodeAcc andalso true
              end
          end, true, Recipients),
  
          All andalso MessageAcc
      end, true, FailedToSend),
  
      AllSentAndReceived andalso FailedToSendNotReceived;
  node_postcondition(_NodeState, _Command, _Response) ->
      false.
  
  