code: protected Set<?> getPrevExpandedRows() {
  		Set<?> keys = getComponentModel().prevExpandedRows;
  		if (keys == null) {
  			return Collections.emptySet();
  		} else {
  			return Collections.unmodifiableSet(keys);
  		}
  	}
docs: Return the row keys that have been expanded.
  <p>
  Note - Only used for when the table is editable.
  </p>
  
  @return the previously expanded row keys.