/// <summary>
/// Returns a copy of this {@code MultiCurrencyAmount} with the specified amount added.
/// <para>
/// This adds the specified amount to this monetary amount, returning a new object.
/// If the currency is already present, the amount is added to the existing amount.
/// If the currency is not yet present, the currency-amount is added to the map.
/// The addition uses standard {@code double} arithmetic.
/// </para>
/// <para>
/// This instance is immutable and unaffected by this method.
///
/// </para>
/// </summary>
/// <param name="amountToAdd"> the amount to add </param>
/// <returns> an amount based on this with the specified amount added </returns>
public MultiCurrencyAmount plus(MultiCurrencyAmount amountToAdd)
{
ArgChecker.notNull(amountToAdd, "amountToAdd");
return(Stream.concat(amounts.stream(), amountToAdd.stream()).collect(toMultiCurrencyAmount()));
}
