code: protected MergingWindowSet<W> getMergingWindowSet() throws Exception {
  		@SuppressWarnings("unchecked")
  		MergingWindowAssigner<? super IN, W> mergingAssigner = (MergingWindowAssigner<? super IN, W>) windowAssigner;
  		return new MergingWindowSet<>(mergingAssigner, mergingSetsState);
  	}
docs: Retrieves the {@link MergingWindowSet} for the currently active key.
  The caller must ensure that the correct key is set in the state backend.
  
  <p>The caller must also ensure to properly persist changes to state using
  {@link MergingWindowSet#persist()}.