You are given a code:

def main_solution(x):
    return func_9(func_13(x, '_'))

Can you predict the output of `main_solution("usozgnreg")` 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": "usozgnreg"}, "ref_output": "u_s_o_z_g_n_r_e_gg_e_r_n_g_z_o_s_u", "ref_code": "def main_solution(x):\n    return func_9(func_13(x, '_'))", "funcname": "main_solution"}
rule