def setETag(etag):
    """
        Set an C{entity tag} for the outgoing response.

        That's "entity tag" as in the HTTP/1.1 I{ETag} header, "used for
        comparing two or more entities from the same requested resource."

        If I am a conditional request, I may modify my response code to
        L{NOT_MODIFIED<http.NOT_MODIFIED>} or
        L{PRECONDITION_FAILED<http.PRECONDITION_FAILED>}, if appropriate for the
        tag given.

        @param etag: The entity tag for the resource being returned.
        @type etag: L{str}

        @return: If I am a C{If-None-Match} conditional request and the tag
            matches one in the request, I return L{CACHED<http.CACHED>} to
            indicate that you should write no body.  Otherwise, I return a
            false value.
        """
