private void addDescendants(final ExplorerNode ancestor, final ExplorerNode descendant) {
        if (ancestor != null) {
            descendants.computeIfAbsent(ancestor, k -> new HashSet<>()).add(descendant);
            addDescendants(getParent(ancestor), descendant);
        }
    }
Get the set of selected items.

@return the set of selected items
stroom-core-client/src/main/java/stroom/explorer/client/presenter/TickBoxSelectionModel.java