You are given a code:

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

Can you predict the output of `main_solution("png")` 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": "png"}, "ref_output": "p_h_n_y_g_f_b_g_z", "ref_code": "def main_solution(x):\n    return func_13(func_20(x, 'hyfbgz'), '_')", "funcname": "main_solution"}
rule