use std::{fs, io, io::prelude::*, path::PathBuf};fn mess_in_the_middle(path: PathBuf) -> io::Result<()> {    let mut file = fs::OpenOptions::new().read(false).write(true).open(path)?;    file.seek(io::SeekFrom::Start(file.metadata()?.len() / 2))?;    file.write(b"hello")?;    Ok(())} fn main() -> Result<(), Box<dyn std::error::Error>> {    let mut args = std::env::args().skip(1);    let scmd = args.next().expect("sub command");    match &*scmd {        "mess-in-the-middle" => mess_in_the_middle(PathBuf::from(args.next().expect("path to file to mess with")))?,        _ => unimplemented!("Unknown subcommand: {}", scmd),    };    Ok(())}