code: public SortedGrouping<T> withPartitioner(Partitioner<?> partitioner) {
  		Preconditions.checkNotNull(partitioner);
  
  		getKeys().validateCustomPartitioner(partitioner, null);
  
  		this.customPartitioner = partitioner;
  		return this;
  	}
docs: Uses a custom partitioner for the grouping.
  
  @param partitioner The custom partitioner.
  @return The grouping object itself, to allow for method chaining.