public function validateDirective($d)
    {
        $id = $d->id->toString();
        $this->context[] = "directive '$id'";
        $this->validateId($d->id);

        $this->with($d, 'description')
            ->assertNotEmpty();

        // BEGIN - handled by InterchangeBuilder
        $this->with($d, 'type')
            ->assertNotEmpty();
        $this->with($d, 'typeAllowsNull')
            ->assertIsBool();
        try {
            // This also tests validity of $d->type
            $this->parser->parse($d->default, $d->type, $d->typeAllowsNull);
        } catch (HTMLPurifier_VarParserException $e) {
            $this->error('default', 'had error: ' . $e->getMessage());
        }
        // END - handled by InterchangeBuilder

        if (!is_null($d->allowed) || !empty($d->valueAliases)) {
            // allowed and valueAliases require that we be dealing with
            // strings, so check for that early.
            $d_int = HTMLPurifier_VarParser::$types[$d->type];
            if (!isset(HTMLPurifier_VarParser::$stringTypes[$d_int])) {
                $this->error('type', 'must be a string type when used with allowed or value aliases');
            }
        }

        $this->validateDirectiveAllowed($d);
        $this->validateDirectiveValueAliases($d);
        $this->validateDirectiveAliases($d);

        array_pop($this->context);
    }