private void modifyState(final ExplorerNode item, final TickBoxState state) {
        final TickBoxState currentState = getState(item);
        if (currentState != state) {
            stateChanges.add(item);
            if (state == null || state == TickBoxState.UNTICK) {
                stateMap.remove(item);
            } else {
                stateMap.put(item, state);
            }
        }
    }
Get the set of selected items.

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