anchor.diff:  
  --type callback_mod() :: atom.
  +-type callback_mod() :: atom().
   -type callback_args() :: any().
   -type callback() :: {callback_mod(), callback_args()}.
   
  
positive.code: }).
  
  -record(req, {
            method :: 'OPTIONS' | 'GET' | 'HEAD' | 'POST' |
                      'PUT' | 'DELETE' | 'TRACE',
            path :: [binary()],
            args,
            raw_path :: binary(),
            version,
            headers :: headers(),
            body :: body(),
            pid :: pid(),
  
negative.code: -type headers() :: [header()].
  -type body() :: binary() | iolist().
  -type response() :: iolist().
  -type http_method() :: 'OPTIONS' | 'GET' | 'HEAD' | 'POST' |
                         'PUT' | 'DELETE' | 'TRACE'.
  -type response_code() :: 100..999.
  -type connection_token_atom() :: keep_alive | close.
  
  -define(l2i(L), list_to_integer(L)).
  