private function createClassesNodeDefinition()
    {
        $classesNode = new ArrayNodeDefinition('classes');

        $classesNode
            ->addDefaultsIfNotSet()
            ->children()
                ->scalarNode('process_handler')
                    ->defaultValue('Lexik\Bundle\WorkflowBundle\Handler\ProcessHandler')
                ->end()
                ->scalarNode('process')
                    ->defaultValue('Lexik\Bundle\WorkflowBundle\Flow\Process')
                ->end()
                ->scalarNode('step')
                    ->defaultValue('Lexik\Bundle\WorkflowBundle\Flow\Step')
                ->end()
            ->end()
        ;

        return $classesNode;
    }