Loading configuration and set parameters?

Question

symfony

i need to have access to only one configuration section in services
, this section is called conf_key and lies under someconfig

class myExtension extends Extension implements PrependExtensionInterface
{ 
   public function prepend(ContainerBuilder $container)
   {
      $container->setParameter("myParam4FutureInjection", $container->getExtensionConfig("someconfig")[1]["conf_key"]);
  }
}

but depending on how many configs are loaded, here $container->getExtensionConfig("someconfig") I get an array of configs, those are not yet merged, and it's not clear where exactly the conf_key key is located each time it can be different
how to do it in the mind ?