use! _ = acquireResourceAsync()

builder.Services.AddDbContext<AppDb>(opt =>
    opt.UseNpgsql(connString));

type U =
    | A
    | B
    module M =  // на самом деле не вложенный модуль
        let f () = ()

type U =
    | A
    | B

module M =
    let f () = ()

public class Solution {
    private static readonly string[] map = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

    public IList<string> LetterCombinations(string digits) {
        var result = new List<string>();
        if(string.IsNullOrEmpty(digits))
            return result;
        Backtrack(digits, 0, new StringBuilder(), result);
        return result;
    }

    private void Backtrack(string digits, int index, StringBuilder current, List<string> result) {
        if(index == digits.Length) {
            result.Add(current.ToString());
            return;
        }

        string letters = map[digits[index] - '0'];
        foreach(char c in letters) {
            current.Append(c);
            Backtrack(digits, index + 1, current, result);
            current.Length--;
        }
    }
}

{ 1..10 } |> List.ofSeq  // неявная последовательность

seq { 1..10 } |> List.ofSeq