code: protected Set<String> getPrevExpandedRows() {
  		Set<String> prev = getComponentModel().prevExpandedRows;
  		if (prev == null) {
  			return Collections.emptySet();
  		} else {
  			return Collections.unmodifiableSet(prev);
  		}
  	}
docs: Return the item ids that have been expanded.
  <p>
  Note - Only used for when the tree is in LAZY mode
  </p>
  
  @return the previously expanded item ids