/// <summary>
/// Debits from the account.
/// </summary>
/// <param name="account">Account.</param>
/// <param name="amount">Debit amount.</param>
public static void Whithdraw(this Account account, decimal amount)
{
if (amount < 0)
{
throw new ArgumentException(nameof(amount));
}
account.Balance -= amount;
account.BenefitPoint -= account.CalculateBenefitPoint(amount);
}