index.cs

// map
from order in docs.Orders
from line in order.Lines
select new {
	Product = line.Product,
	Total = ((line.Quantity * line.PricePerUnit) * (1 - line.Discount))
}

// reduce
from result in results
group result by result.Product into g
select new {
	Product = g.Key,
	Total = g.Sum(x => x.Total)
}