struct BST {
    l: Option< Box<BST> >,
    r: Option< Box<BST> >,
    v: i32,
}

impl BST {
    fn new(x: i32) -> Self {
        BST { l: None, r: None, v:x }
    }

    fn add(&mut self, x: i32) {
        if self.v < x {
            if let Some(ref mut t) = self.l {
                t.add(x);
            } else {
                self.l = Some( Box::new(BST::new(x)) );
            }
        }
        if self.v > x {
            if let Some(ref mut t) = self.r {
                t.add(x);
            } else {
                self.r = Some( Box::new(BST::new(x)) );
            }
        }
    }
}

fn main() {
    let mut b = BST::new(5);
    b.add(4);
    b.add(6);
}