anchor.diff:  
   
   -spec report_error(rcl_state:t(), error()) -> none() | error().
  -report_error(State, Error={error, {Module, Reason}}) ->
  -    io:format("~s~n", [Module:format_error(Reason)]),
  +report_error(State, Error) ->
  +    io:format(format_error(Error)),
       usage(),
       case rcl_state:caller(State) of
           command_line ->
  
positive.code:         {ok, State1} ->
              {ok, State1};
          E={error, _} ->
              report_error(State0, E)
      end.
  
  -spec usage() -> ok.
  
negative.code: -spec format_error(Reason::term()) -> iolist().
  format_error({invalid_return_value, Provider, Value}) ->
      [rcl_provider:format(Provider), " returned an invalid value ",
       io_lib:format("~p", [Value])];
  format_error({error, {Module, Reason}}) ->
      io_lib:format("~s~n", [Module:format_error(Reason)]).
  
  
  %%============================================================================
  %% internal api
  