class SpecialInteger {
    constructor(x = null) {
        this.undo_manager = new UndoManager(this);
        this.set(x);
    }


    set(x = null) {
        this.data = x;
    }

    get() {
        return this.data;
    }
}

let i = new SpecialInteger(5);
i.set(8);
i.set(4);
i.set(3);

i.undo();
i.undo();

console.log(i.get());