You are given a code:

def main_solution(x):
    return func_4(func_17('usol'))

Can you predict the output of `main_solution("exf")` 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": "exf"}, "ref_output": "losu", "ref_code": "def main_solution(x):\n    return func_4(func_17('usol'))", "funcname": "main_solution"}
rule