public ICommand ChangePositionCommand => new Command((obj) =>
        {
            _positionIndex++;
            if (_positionIndex >= Positions.Count)
                _positionIndex = 0;
            Position = Positions[_positionIndex];
            RaisePropertyChanged(nameof(Position));
        });

public ICommand ChangePositionCommand {
    return new Command((obj) => {
        _positionIndex++;
        if (_positionIndex >= Positions.Count)
            _positionIndex = 0;
        Position = Positions[_positionIndex];
        RaisePropertyChanged(nameof(Position));
    });
}

public int X { get {return A + B;}}

public int X => A + B;