code: public Set<Resource<L>> getChildren(Resource<L> resource) {
          graphLockRead.lock();
          try {
              Set<Resource<L>> directChildren = neighborIndex.successorsOf(resource);
              return Collections.unmodifiableSet(new HashSet<>(directChildren));
          } finally {
              graphLockRead.unlock();
          }
      }
docs: Returns an immutable {@link Set} of the direct children of the given resource.
  
  @param resource the resource whose children are to be returned
  
  @return a {@link Set} if direct children of the given resource