private ImmutableList <Product> MergeProducts(Subset subset, string name, ImmutableList <Product> childProducts, ImmutableList <Product> products)
{
var mergedProducts =
from product in products
let matchingChildProducts = (
from childProduct in childProducts
where subset.Of(childProduct).Equals(subset.Of(product))
select childProduct
).ToImmutableList()
select product.With(name, new CollectionElement(matchingChildProducts));
return(mergedProducts.ToImmutableList());
}