You are given a code:

def main_solution(x):
    return (func_13(x, '_') + func_13('xukax', '_'))

Can you predict the output of `main_solution("xitrvnmuo")` 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": "xitrvnmuo"}, "ref_output": "x_i_t_r_v_n_m_u_ox_u_k_a_x", "ref_code": "def main_solution(x):\n    return (func_13(x, '_') + func_13('xukax', '_'))", "funcname": "main_solution"}
rule