public function getScheme($scheme, $config, $context)
    {
        if (!$config) {
            $config = HTMLPurifier_Config::createDefault();
        }

        // important, otherwise attacker could include arbitrary file
        $allowed_schemes = $config->get('URI.AllowedSchemes');
        if (!$config->get('URI.OverrideAllowedSchemes') &&
            !isset($allowed_schemes[$scheme])
        ) {
            return;
        }

        if (isset($this->schemes[$scheme])) {
            return $this->schemes[$scheme];
        }
        if (!isset($allowed_schemes[$scheme])) {
            return;
        }

        $class = 'HTMLPurifier_URIScheme_' . $scheme;
        if (!class_exists($class)) {
            return;
        }
        $this->schemes[$scheme] = new $class();
        return $this->schemes[$scheme];
    }