You are given a code:

def main_solution(x):
    return func_13(func_20('dfujxz', x), '_')

Can you predict the output of `main_solution("mfqxulfvug")` 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": "mfqxulfvug"}, "ref_output": "d_m_f_f_u_q_j_x_x_u_z_l_f_v_u_g", "ref_code": "def main_solution(x):\n    return func_13(func_20('dfujxz', x), '_')", "funcname": "main_solution"}
rule