anchor.diff:          , allowed_methods/2
           , resource_exists/2
           , delete_resource/2
  +        , content_types_accepted/2
           , content_types_provided/2
           , to_json/2
           , process_post/2
  +        , process_put/2
           ]
          ).
   
  
positive.code:         {error, _} ->
              {false, Req, State}
      end.
  
  content_types_provided(Req, State) ->
      {[{"application/json", to_json}], Req, State}.
  
negative.code:     {false, Req, State}.
  
  allowed_methods(Req, State) ->
      {['GET', 'POST', 'PUT', 'DELETE', 'HEAD'], Req, State}.
  
  resource_exists(Req, State = #state{mode = collection}) ->
      {true, Req, State};
  