@SuppressWarnings("unchecked")
  static void register(BeanDefinitionRegistry registry,
                       Class<? extends SpockMockPostprocessor> postProcessor,
                       Set<Definition> definitions) {
    SpyPostProcessor.register(registry);
    BeanDefinition definition = getOrAddBeanDefinition(registry, postProcessor);
    ValueHolder constructorArg = definition.getConstructorArgumentValues().getIndexedArgumentValue(0, Set.class);
    Set<Definition> existing = (Set<Definition>)constructorArg.getValue();
    if (definitions != null) {
      existing.addAll(definitions);
    }
  }
Register the processor with a {@link BeanDefinitionRegistry}. Not required when
using the Spocks {@link spock.lang.Specification} as registration is automatic.

@param registry the bean definition registry
@param postProcessor the post processor class to register
@param definitions the initial mock/spy definitions
spock-spring/src/main/java/org/spockframework/spring/mock/SpockMockPostprocessor.java