code: public Map<K, V> loadAll(Iterable<? extends K> keys, Executor executor) throws Exception {
      throw new UnsupportedOperationException();
    }
docs: Loads multiple values to the cache.
  
  <p>From inside this method it is illegal to call methods on the same cache. This
  may cause a deadlock.
  
  <p>The method is provided to complete the API. At the moment cache2k is not
  using it. Please see the road map.
  
  @param keys set of keys for the values to be loaded
  @param executor an executor for concurrent loading
  @return The loaded values. A key may map to {@code null} if the cache permits {@code null} values.
  @throws Exception Unhandled exception from the loader. Exceptions are suppressed or
  wrapped and rethrown via a {@link CacheLoaderException}.
  If an exception happens the cache may retry the load with the
  single value load method.