public GSM(string model, string manufacturer, decimal?price = null, string owner = null, Battery battery = null, Display display = null)
{
this.Model = model;
this.Manufacturer = manufacturer;
this.Price = price;
this.Owner = owner;
this.Battery = battery;
this.Display = display;
this.CallHistory = callHistory;
}
public GSM(string model, string manufacturer, decimal?price, string owner, Battery battery)
: this(model, manufacturer, price, owner, battery, null)
{
}