[Vala] Widgets hierarchy, or how get label on the bix?

[Александер Пономарёв <shootnix gmail com>]

Hi all,
please, tell me somebody, is possible in the vala to get access to one
widget trough another one?
To be clear: I have a box and one label on it:

    var box = new Box(...);
    var label = new Label(...);
    box.pack_start(label, ...);

and now I have to get access to this label using the box:

    box.label.get_text() // or something like that.

Or more clear:

    public void set_box(Box box) {
        string label_title = "Hello, Vala!";
        box.label.set_text(label_title);
    }

How can I solve this?