Eric Lippert's challenge “comma-quibbling”, best answer?

Question

I wanted to bring this challenge to the attention of the stackoverflow community. The original problem and answers are here. BTW, if you did not follow it before, you should

Answer 1

It hasn't quite been a decade since the last post so here's my variation:

    public static string CommaQuibbling(IEnumerable items)
    {
        var text = new StringBuilder();
        string sep = null;
        int last_pos = items.Count();
        int next_pos = 1;

        foreach(string item in items)
        {
            text.Append($"{sep}{item}");
            sep = ++next_pos < last_pos ? ", " : " and ";
        }

        return $"{{{text}}}";
    }