'Searches for new URLs by adding and subtracting numbers to the file
and the parameters.
:param fuzzable_request: A fuzzable_request instance that contains
(among other things) the URL to test.'

def crawl(self, fuzzable_request):

    if (fuzzable_request.get_data() or (fuzzable_request.get_method() != 'GET')):
        return
    url = fuzzable_request.get_url()
    headers = Headers([('Referer', url.url_string)])
    fuzzable_request.get_headers().update(headers)
    original_response = self._uri_opener.send_mutant(fuzzable_request, cache=True)
    if (original_response.is_text_or_html() or self._fuzz_images):
        fr_generator = self._mangle_digits(fuzzable_request)
        response_repeater = repeat(original_response)
        args = izip(fr_generator, response_repeater)
        self.worker_pool.map_multi_args(self._do_request, args)
        self._already_visited.add(fuzzable_request.get_uri())
