You are given a code:

def main_solution(x):
    return func_20(func_15('ezqy'), func_13(x, '_'))

Can you predict the output of `main_solution("nmfbn")` 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": "nmfbn"}, "ref_output": "\u00abne_\u00bbm\u00ab_zf\u00bb_\u00abbq_\u00bbn\u00aby\u00bb", "ref_code": "def main_solution(x):\n    return func_20(func_15('ezqy'), func_13(x, '_'))", "funcname": "main_solution"}
rule