Binding selected items in nested ListBox with Sele

I'm developing a Windows Phone 8.1 app with MVVM pattern (I'm using Prism as a framework)

The goal is to get the selected items in a few lists, like that:

I have following XAML:

    <ListView 
    x:Name="abc"
    ItemsSource="{Binding Symbols}">
    <ListView.ItemTemplate>
        <DataTemplate>
            <StackPanel>
                <ListBox
                    SelectionMode="Multiple"
                    ItemsSource="{Binding List}"
Extensions:ListViewExtensions.BindableSelection="{Binding SelectedItems, ElementName=abc, Mode=TwoWay}">
                    ...

and in the ViewModel:

    public List<SymbolsGroupViewModel> Symbols {
        get { return _symbols ?? (_symbols = _dataService.Symbols.GetGroupViewModels()); }
        set { _symbols = value; }
    }

where class SymbolsGroupViewModel looks like that:

public class SymbolsGroupViewModel : ViewModel
{
    private readonly INavigationService _navigationService;
    private readonly DataService _dataService;

    private ObservableCollection<Symbol> _selectedItems = new ObservableCollection<Symbol>();

    public SymbolsGroupViewModel(INavigationService navigationService, DataService dataService)
    {
        _navigationService = navigationService;
        _dataService = dataService;
    }

    public SymbolsGroupViewModel() { }

    public Symbol Header { get; set; }

    public List<Symbol> List { get; set; }

    public ObservableCollection<Symbol> SelectedItems
    {
        get { return _selectedItems; }
        set { SetProperty(ref _selectedItems, value); }
    }
}

It is supposed to use the BindableSelection extension from WinRT XAML Toolkit, like here: https://stackoverflow.com/a/25430935/5194338

It works for me with NOT nested lists, however when I adapt the solution to nested list, the SelectedItems lists contain 0 elements.

Does anybody know if it is possible to use this extension in nested lists and if it is, what am I doing wrong?

Thank you for your help.

I've found where the mistake was so I'll leave the answer in case someone had similar problem...

In the nested list the binding should have been written as below:

Extensions:ListBoxExtensions.BindableSelection="{Binding SelectedItems, Mode=TwoWay}"

Giving there ElementName was a bad idea
I haven't noticed before, that there are separate extensions for each kind of list. In my case I should have used ListBoxExtensions.

Hope this will help someone in the future.