You are given a code:

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

Can you predict the output of `main_solution("rkd")` 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": "rkd"}, "ref_output": "n_d_k_krkdfgpb", "ref_code": "def main_solution(x):\n    return (func_13('ndkk', '_') + (x + 'fgpb'))", "funcname": "main_solution"}
rule