class Nil {}
type Tree<T> = Nil | ('T, Tree<'T>, Tree<'T>);

let t: Tree<Int> = (
    3, 
    (
        1, 
        Nil(), 
        (
            3, 
            (1, Nil(), Nil()), 
            (2, Nil(), Nil())
        )
    ), 
    (2, Nil(), Nil())
);