You are given a code:

def main_solution(x):
    return func_20(func_11('rzk', 2), func_13(x, '_'))

Can you predict the output of `main_solution("ttbwul")` without writing any code? Please reason and put your final answer in the following json format: {"output": <your output>}, where <your output> should be the final string.
user
{"ref_input": {"x": "ttbwul"}, "ref_output": "ttb_mt_b_w_u_l", "ref_code": "def main_solution(x):\n    return func_20(func_11('rzk', 2), func_13(x, '_'))", "funcname": "main_solution"}
rule