You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.rs
    path of file: ./repos/sigstore-rs/examples/rekor/get_public_key
    
    the code of the file until where you have to start completion: //
// Copyright 2021 The Sigstore Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance w
    
async fn main() {
    /*
    Returns the public key that can be used to validate the signed tree head

    Example command :
    cargo run --example get_public_key
    */

    let matches = Command::new("cmd")
    .arg(Arg::new("tree_id")
             .long("tree_id")
             .value_name("TREE_ID")
             .help("The tree ID of the tree that you wish to prove consistency for. To use the default value, do not input any value."))
    .arg(Arg::new("output")
            .short('o')
            .long("output")
            .value_name("OUTPUT_FILE")
            .num_args(0..=1)
            .require_equals(true)
            .default_missing_value("key.pub")
            .help("The path to the output file that you wish to store the public key in. To use the default value (pub.key), do not provide OUTPUT_FILE."));

    let flags = matches.get_matches();
    let configuration = Configuration::default();
    let pubkey = pubkey_api::get_public_key(
        &configuration,
        flags.get_one::<String>("tree_id").map(|s| s.as_str()),
    )
    .await
    .expect("Unable to retrieve public key");

    if let Some(out_path) = flags.get_one::<String>("output") {
        match write(out_path, pubkey) {
            Ok(_) => (),
            Err(e) => {
                eprintln!("Could not write to {out_path}: {e}");
                process::exit(1);
            }
        }
    } else {
        print!("{}", pubkey);
    }
}