anchor.diff:    ,export = undefined :: _               %% resource exports
     ,mthd   = undefined :: _
     ,uri    = undefined :: uri:uri()
  -  ,head   = undefined :: _
  +  ,inhead = undefined :: _               %% ingress http headers
  +  ,eghead = undefined :: _               %% egress http headers
     ,env    = undefined :: _
   }).
   
  
positive.code: %%
  -spec is_resource_exists(#rest{}) -> {ok, #rest{}} | {error, {not_found, _}}.
  
  is_resource_exists(#rest{uri = Uri, env = Env} = Rest) ->
     {_, Accept} = lists:keyfind('Accept', 1, Env),
     case f(Rest, exists, Accept) of
        true ->
           {ok, Rest};
  
negative.code: 
  %%
  %% build a resource context 
  req(#rest{uri = Url, inhead = Head, env = Env}) ->
     {Url, Head, Env}.
  
  %%
  