public function validateId($id)
    {
        $id_string = $id->toString();
        $this->context[] = "id '$id_string'";
        if (!$id instanceof HTMLPurifier_ConfigSchema_Interchange_Id) {
            // handled by InterchangeBuilder
            $this->error(false, 'is not an instance of HTMLPurifier_ConfigSchema_Interchange_Id');
        }
        // keys are now unconstrained (we might want to narrow down to A-Za-z0-9.)
        // we probably should check that it has at least one namespace
        $this->with($id, 'key')
            ->assertNotEmpty()
            ->assertIsString(); // implicit assertIsString handled by InterchangeBuilder
        array_pop($this->context);
    }