code: public Set<ResourceType<L>> getParents(ResourceType<L> resourceType) {
          Set<ResourceType<L>> directParents = index.predecessorsOf(resourceType);
          return Collections.unmodifiableSet(directParents);
      }
docs: Returns the direct parent types of the given resource type.
  
  @param resourceType the type whose parents are to be returned
  
  @return the direct parents of the given resource type