You are given a code:

def main_solution(x):
    return func_13(func_5(x, 'rpex'), '_')

Can you predict the output of `main_solution("hff")` 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": "hff"}, "ref_output": "r_p_e_x_h_f_f", "ref_code": "def main_solution(x):\n    return func_13(func_5(x, 'rpex'), '_')", "funcname": "main_solution"}
rule