Efficient killer/sandwich sudoku calculator in one line of c#

KillerSudoku.cs

void Main()
{
	var x = new List<int>{1,2,3,4,5,6,7,8,9};
	Killer(x,6).Dump();
}

public static List<List<int>> Killer(List<int> alphabet, int target) => 
	alphabet.Where(x=> x <= target).SelectMany(letter=>
		(letter == target) ?
		new List<List<int>> { new List<int> { letter }} :
		Killer(alphabet.Where(x => x > letter).ToList(), target - letter)
		.Select(s => s.Prepend(letter).ToList())
	).ToList();