You are given a code:

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

Can you predict the output of `main_solution("oxsb")` 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": "oxsb"}, "ref_output": "s_y_v_c_i_u_i_m_t_f_v_g", "ref_code": "def main_solution(x):\n    return func_13(func_20('sviitv', 'ycumfg'), '_')", "funcname": "main_solution"}
rule