下面代码中gray_code函数的作用是生成所有长度为n位的格雷码，请问横线上应填写的内容是？

def gray_code(n):
    if n == 0:
        return ["0"]
    if n == 1:
        return ["0", "1"]
    prev = gray_code(n-1)
    result = []
    for s in prev:
        _________________________
    for i in range(len(prev)-1, -1, -1):
        result.append("1" + prev[i])
    return result