class type http_request_header_notification =object..end
 This object notifies the user that a new request header has arrived.
 The header is accessible by the environment object. The channels
 also contained in this object are locked at this moment. The user must
 now either call schedule_accept_body or schedule_reject_body. The
 user will get a second notification (a http_request_notification, below)
 when the request body has completely arrived (in case of acceptance), or
 immediately (in case of rejection). One can also call schedule_finish
 at any time to drop the current request.
method req_state : engine_req_state`Received_header at the
 moment when this notification is delivered.method environment : extended_async_environment`Received_header only the request header and the 
 derived fields are accessible, and the input and output channels are
 locked. In the state `Receiving_body the input channel is unlocked,
 but it is not yet filled (reading from it may cause the exception 
 Buffer_underrun). The output channel remains locked.
 In the state `Received_request, the input channel is unlocked and filled
 with data, and the output channel is unlocked, too.
This environment is not fully CGI-compatible. In particular, the following differences exist:
cgi_path_info and no cgi_path_translated.Status response header works as in CGI. The Location header, however,
   must be a full URL when set (only browser redirects)Content-Length is not set. In CGI this is interpreted as missing body.
   It is unlikely that clients send requests with chunked encoding, as this
   may cause interoperability problems anyway.
method schedule_accept_body : on_request:(http_request_notification -> unit) ->
       ?on_error:(unit -> unit) -> unit -> unit
 This method returns immediately, and sets callbacks for certain events.
 When the body has completely arrived (or is empty), the function 
 on_request is called back. The argument is the full request notification
 (see below).
 When the request is dropped for some reason, on_error is called back instead.
 This can be used to free resources, for example.
 Neither of the callbacks must raise exceptions.
method schedule_reject_body : on_request:(http_request_notification -> unit) ->
       ?on_error:(unit -> unit) -> unit -> unit
 This method returns immediately, and sets callbacks for certain events.
 When the body has completely arrived (or is empty), the function 
 on_request is called back. The argument is the full request notification
 (see below).
 When the request is dropped for some reason, on_error is called back instead.
 This can be used to free resources, for example.
 Neither of the callbacks must raise exceptions.
method schedule_finish : unit -> unit