You are given a code:

def main_solution(x):
    return func_20(('vtf' + x), func_15(x))

Can you predict the output of `main_solution("vifgbddlj")` 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": "vifgbddlj"}, "ref_output": "v\u00abtvf\u00bbv\u00abiif\u00bbg\u00abbfd\u00bbd\u00ablgj\u00bb\u00abb\u00bb\u00abd\u00bb\u00abd\u00bb\u00abl\u00bb\u00abj\u00bb", "ref_code": "def main_solution(x):\n    return func_20(('vtf' + x), func_15(x))", "funcname": "main_solution"}
rule