anchor.diff:  node_postcondition(_State, _Command, {error, {primary, _Node}}) ->
       %% Ignore failures from contacting the wrong node -- should this be a precondition?
       true;
  -node_postcondition(_State, _Command, _Response) ->
  +node_postcondition(_State, {call, ?MODULE, _Fun, [Node|_Rest]}=Command, Response) ->
  +    node_debug("node ~p: failed postcondition for command: ~p response: ~p", [Node, Command, Response]),
  +    false;
  +node_postcondition(_State, Command, Response) ->
  +    node_debug("fallthrough postcondition failed for command: ~p response: ~p", [Command, Response]),
       false.
   
   %%%===================================================================
  
positive.code: node_postcondition(_State, {call, ?MODULE, write, [Node, Key, Value]}, ok) ->
      node_debug("node ~p: writing key ~p with value ~p", [Node, Key, Value]),
      true;
  node_postcondition(#state{store=Store}=_State, {call, ?MODULE, read, [Node, Key]}, {ok, Value}) ->
      case dict:find(Key, Store) of 
          {ok, Value} ->
  
negative.code: %%%===================================================================
  
  write(Node, Key, Value) ->
      case rpc:call(?NAME(Node), ?PB_MODULE, write, [Key, Value]) of 
          {badrpc, {'EXIT', Error}} ->
              Error;
          Other ->
              Other
      end.
  
  read(Node, Key) ->
      rpc:call(?NAME(Node), ?PB_MODULE, read, [Key]).
  