public UserIndex Update(IAssignedUserOrder assignedUserOrder)
{
var index = MyOrders.IndexOf(assignedUserOrder);
if (index < 0)
{
throw new InvalidOperationException(string.Format("MyOrders does not contains user order {0}.", assignedUserOrder));
}
var newMyOrders = MyOrders.SetItem(index, assignedUserOrder);
return(new UserIndex(User, LastTransaction, newMyOrders, AssigningOrders));
}