    """HTTP servlets"""

    from time import gmtime, strftime

    from Servlet import Servlet

    class HTTPServlet(Servlet):
        """A HTTP servlet.

        HTTPServlet implements the respond() method to invoke methods such as
        respondToGet() and respondToPut() depending on the type of HTTP request.
        Example HTTP request methods are GET, POST, HEAD, etc.
        Subclasses implement HTTP method FOO in the Python method respondToFoo.
        Unsupported methods return a "501 Not Implemented" status.

        Note that HTTPServlet inherits awake() and respond() methods from
        Servlet and that subclasses may make use of these.

        See also: Servlet

        FUTURE
          * Document methods (take hints from Java HTTPServlet documentation)

        """

        ## Init ##

        def __init__(self):
            Servlet.__init__(self)
            self._methodForRequestType = {} # a cache; see respond()

        ## Transactions ##

        def respond(self, trans):
            """Respond to a request.

            Invokes the appropriate respondToSomething() method depending on the
            type of request (e.g., GET, POST, PUT, ...).

            """
            request = trans.request()
            httpMethodName = request.method()
            # For GET and HEAD, handle the HTTP If-Modified-Since header:
            # if the object's last modified time is the same
            # as the IMS header, we're done.
            if httpMethodName in ('GET', 'HEAD'):
                lm = self.lastModified(trans)
                if lm:
                    lm = strftime('%a, %d %b %Y %H:%M:%S GMT', gmtime(lm))
                    trans.response().setHeader('Last-Modified', lm)
                    ims = request.environ(
                        ).get('HTTP_IF_MODIFIED_SINCE') or request.environ(
                        ).get('IF_MODIFIED_SINCE')
                    if ims and ims.split(';', 1)[0] == lm:
                        trans.response().setStatus(304, 'Not Modified')
                        return
            method = self._methodForRequestType.get(httpMethodName)
            if not method:
                methName = 'respondTo' + httpMethodName.capitalize()
                method = getattr(self, methName, self.notImplemented)
                self._methodForRequestType[httpMethodName] = method
            method(trans)

        @staticmethod
        def notImplemented(trans):
            trans.response().setStatus(501, 'Not Implemented')

        @staticmethod
        def lastModified(trans):
            """Get time of last modification.

            Return this object's Last-Modified time (as a float),
            or None (meaning don't know or not applicable).

            """
            return None

        def respondToHead(self, trans):
            """Respond to a HEAD request.

            A correct but inefficient implementation.

            """
            res = trans.response()
            w = res.write
            res.write = lambda *args: None
            self.respondToGet(trans)
            res.write = w