code: public Set<ResourceType<L>> getChildren(ResourceType<L> resourceType) {
          Set<ResourceType<L>> directChildren = index.successorsOf(resourceType);
          return Collections.unmodifiableSet(directChildren);
      }
docs: Returns the direct child types of the given resource type.
  
  @param resourceType the type whose children are to be returned
  
  @return the direct children of the given resource type