code: @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);
      }
    }
docs: 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