public IAction Lose(Province what) => new IncomeChange(PlayerId, -what.Earnings);
public bool IsAllyOf(Province prov) => Army.IsAllyOf(prov.Army);
public IAction Gain(Province what) => new IncomeChange(PlayerId, what.Earnings);
public virtual Province StartMove(Province dest, Army army) => WithArmy(Army.Subtract(army));