/// <summary>
/// This constructor shall be used when there is promotion.
/// </summary>
/// <param name="promotionEngine"></param>
/// <param name="cartEngine"></param>
public Billing(IPromotionEngine <List <char> > promotionEngine, ICartEngine <List <char> > cartEngine)
{
this.promotionEngine = promotionEngine;
this.cartEngine = cartEngine;
}
/// <summary>
/// This constructor shall be used when there is no promotion.
/// </summary>
/// <param name="cartEngine"></param>
public Billing(ICartEngine <List <char> > cartEngine)
{
this.cartEngine = cartEngine;
}