private void verifyGlobalExtension(Class<?> clazz) {
    if (!IGlobalExtension.class.isAssignableFrom(clazz))
      throw new ExtensionException(
        "Class '%s' is not a valid global extension because it is not derived from '%s'"
      ).withArgs(clazz.getName(), IGlobalExtension.class.getName());
  }
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