@Override
  public <T extends IStatelessAnnotationDrivenExtension<?>> T getStatelessAnnotationDrivenExtension(Class<T> extensionClass) {
    return extensionClass.cast(statelessExtensions.computeIfAbsent(extensionClass, this::instantiateAndConfigureExtension));
  }
Maintains a registry of global Spock extensions and their configuration objects,
which can be used to configure other extensions.

@author Peter Niederwieser
spock-core/src/main/java/org/spockframework/runtime/GlobalExtensionRegistry.java