Can't get coordinates of widget within group in fltk-rs

groupCoords.rs

use fltk::*;

fn main() {
    let app = app::App::default();
    let mut win = window::Window::default().with_size(400, 300);

    let mut hpack = group::Pack::default().with_size(400, 100);
    hpack.set_spacing(10);
    hpack.set_type(group::PackType::Horizontal);

    let mut b1 = button::Button::new(0,0,100,100,"");
    let mut b2 = button::Button::new(0,0,100,100,"");
    let mut b3 = button::Button::new(0,0,100,100,"");

    // println!("{}", coords(b1.x, b2.y));


    // b1.set_label();
    hpack.end();
    hpack.auto_layout();

    b1.set_label(&coords(b1.x(), b1.y()));
    b2.set_label(&coords(b2.x(), b2.y()));
    b3.set_label(&coords(b3.x(), b3.y()));

    fn coords(x: i32, y: i32) -> String{
        let myx = x.to_string(); 
        let myy = y.to_string(); 
        let l = vec!["(", &myx, ",", &myy, ")"];
        let j = l.join("");
        return j; 
    }
    win.end();
    win.show();

    app.run().unwrap();
}