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: scell_seal_string_echo.rs
    path of file: ./repos/themis/tools/rust
    
    the code of the file until where you have to start completion: // Copyright 2019 (c) rust-themis developers
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#![allow(clippy::needless_borrow)]

use std::process::exit;

use clap::clap_app;
use themis::secure_cell::SecureCell;

fn main() {
    let matches = clap_app!(scell_seal_string_echo =>
        (version: env!("CARGO_PKG_VERSION"))
        (about: "Secure Cell echo testing tool (sealing mode).")
        (@arg mode:    +required "<enc|dec>")
        (@arg key:     +required "master key")
        (@arg message: +required "message to encrypt or decrypt")
        (@arg context:           "user context")
    )
    .get_matches();

    let mode = matches.value_of("mode").unwrap();
    let key = matches.value_of("key").unwrap();
    let message = matches.value_of("mes
    
fn main() {
    let matches = clap_app!(scell_seal_string_echo =>
        (version: env!("CARGO_PKG_VERSION"))
        (about: "Secure Cell echo testing tool (sealing mode).")
        (@arg mode:    +required "<enc|dec>")
        (@arg key:     +required "master key")
        (@arg message: +required "message to encrypt or decrypt")
        (@arg context:           "user context")
    )
    .get_matches();

    let mode = matches.value_of("mode").unwrap();
    let key = matches.value_of("key").unwrap();
    let message = matches.value_of("message").unwrap();
    let context = matches.value_of("context").unwrap_or_default();

    let cell = SecureCell::with_key(key)
        .unwrap_or_else(|_| {
            eprintln!("invalid parameters: empty master key");
            exit(1);
        })
        .seal();

    match mode {
        "enc" => {
            let encrypted = cell
                .encrypt_with_context(message, context)
                .unwrap_or_else(|error| {
                    eprintln!("failed to encrypt message: {error}");
                    exit(1);
                });
            println!("{}", base64::encode(&encrypted));
        }
        "dec" => {
            let decoded_message = base64::decode(&message).unwrap_or_else(|error| {
                eprintln!("failed to decode message: {error}");
                exit(1);
            });
            let decrypted = cell
                .decrypt_with_context(decoded_message, context)
                .unwrap_or_else(|error| {
                    eprintln!("failed to decrypt message: {error}");
                    exit(1);
                });
            println!("{}", std::str::from_utf8(&decrypted).expect("UTF-8 string"));
        }
        other => {
            eprintln!("invalid mode {other}, use \"enc\" or \"dec\"");
            exit(1);
        }
    }
}