You are given a code:

def main_solution(x):
    return (func_13(x, '_') + func_15('yxj'))

Can you predict the output of `main_solution("viylwuuz")` 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": "viylwuuz"}, "ref_output": "v_i_y_l_w_u_u_z\u00aby\u00bb\u00abx\u00bb\u00abj\u00bb", "ref_code": "def main_solution(x):\n    return (func_13(x, '_') + func_15('yxj'))", "funcname": "main_solution"}
rule