code: protected void addPrevExpandedCurrent() {
  		Set<String> rows = getExpandedRows();
  		if (!rows.isEmpty()) {
  			WTreeComponentModel model = getOrCreateComponentModel();
  			if (model.prevExpandedRows == null) {
  				model.prevExpandedRows = new HashSet<>();
  			}
  			model.prevExpandedRows.addAll(rows);
  		}
  	}
docs: Save the currently open rows.
  <p>
  Note - Only used for when the tree is in LAZY mode
  </p>