public virtual Amount Subtract(Amount other) =>
other == null ? throw new ArgumentNullException(nameof(other))
: other.Currency != this.Currency ? throw new ArgumentException("Mismatched currency.")
: other.Value > this.Value ? throw new ArgumentException("Insufficient funds.")
: new Amount(this.Currency, this.Value - other.Value);
public Money PayableAt(Money money, Amount expense, Timestamp time) => money is GiftCard gift && gift.ValidBefore.CompareTo(time) < 0 ? Amount.Zero(expense.Currency)
public virtual Amount Add(Amount other) =>
other == null ? throw new ArgumentNullException(nameof(other))
: other.Currency != this.Currency ? throw new ArgumentException("Mismatched currency.")
: new Amount(this.Currency, this.Value + other.Value);