symfony/tests/Symfony/Tests/Components/DependencyInjection/Fixtures/xml/services5.xml

<?xml version="1.0" ?>

<container xmlns="http://www.symfony-project.org/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.symfony-project.org/schema/dic/services http://www.symfony-project.org/schema/dic/services/services-1.0.xsd">
  <services>
    <service id="foo" class="FooClass">
      <argument type="service">
        <service class="BarClass">
          <argument type="service">
            <service class="BazClass">
            </service>
          </argument>
        </service>
      </argument>
    </service>
  </services>
</container>