重複順列（再帰）

Generate.cs

// using System.Collections.Generic;

static IEnumerable<string> Generate(string[] keyword, int length)
{
    if (length > 1)
    {
        foreach (var key in keyword)
        {
            foreach (var key2 in Generate(keyword, length - 1))
            {
                yield return key + key2;
            }
        }
    }
    else
    {
        foreach (var key in keyword)
        {
            yield return key;
        }
    }
}