title: Symfony service injection syntax
body: <p>I am working on a Symfony 2 app implemented by another company and came across the following service definition:</p>
  
  <pre><code>service_id:
      class: 'path\to\class'
      calls:
         - [setRequest, ['@?request=']]
  </code></pre>
  
  <p>I know what the question mark in <code>@?request=</code> means (if the service does not exist, <code>setRequest</code> does not get called), but what does the equal sign at the end of  mean?</p>
  
  <p>Thanks!</p>