You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: d.go
    path of file: ./repos/codeforces-go/leetcode/weekly/261/d
    
    the code of the file until where you have to start completion: package main

import "strings"

// github.com/EndlessCheng/codeforces-go
func smallestSubsequence(s string, k int, letter byte, repetition int) string {
	ans :=
    
func smallestSubsequence(s string, k int, letter byte, repetition int) string {
	ans := []byte{}
	unread := strings.Count(s, string(letter)) // 未遍历到的 letter 个数
	inQ := 0                                   // 在单调队列中的 letter 个数
	q := []byte{}
	for i, n := 0, len(s); len(ans) < k; i++ {
		// 将 s[i] 插入单调队列
		for i < n && len(q) > 0 && s[i] < q[len(q)-1] {
			if q[len(q)-1] == letter { // 特判队尾是 letter 的情况
				if inQ+unread <= repetition { // 弹出队尾会导致无法凑足 repetition 个 letter
					break
				}
				inQ--
			}
			q = q[:len(q)-1]
		}
		if i < n {
			if s[i] == letter {
				unread--
				inQ++
			}
			q = append(q, s[i])
		}

		if i >= n-k {
			// 从队首拿到字典序最小的字符
			if q[0] == letter {
				inQ--
				repetition--
				ans = append(ans, q[0])
			} else if len(ans)+repetition < k {
				ans = append(ans, q[0])
			}
			q = q[1:]
		}
	}
	return string(ans)
}