Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / vendor / symfony / dependency-injection / Tests / Fixtures / xml / extensions / services1.xml

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:project="http://www.example.com/schema/project">

    <parameters>
        <parameter key="project.parameter.foo">BAR</parameter>
    </parameters>

    <services>
        <service id="project.service.foo" class="BAR" />
    </services>

    <project:bar babar="babar">
        <another />
        <another2>%project.parameter.foo%</another2>
    </project:bar>

</container>