// cp
let (s, files) = Settings::default().parse(args_os());

let (sources, destination) = if s.no_target_directory {
    // TODO: assert that there are two files and return them
} else if let Some(dest) = s.target_directory {
    (files, dest)
} else {
    // TODO make proper error
    let dest = files.pop().unwrap();
    (files, dest)
}

#[derive(Arguments)]
enum Arg {
   #[arg("-a")]
   Foo,
   #[arg("FILES", ..)]
   Files,
}

#[derive(Default)]
struct Settings {
   foo: bool,
   files: Vec<PathBuf>,
}

impl Options<Arg> for Settings {
   fn apply(&mut self, arg: Arg) {
       match self {
           Arg::Foo => self.foo = true,
           Arg::Files(f) => self.files = f,
       }
   }
}

fn main() {
    let s = Settings::default().parse(args_os());
    println!("{:?}", s.files);
}

#[derive(Arguments)]
enum Arg {
   #[arg("-a")]
   Foo,
}

struct Settings {
   foo: bool,
}

impl Options<Arg> for Settings {
   fn apply(&mut self, arg: Arg) {
       match self {
           Arg::Foo => self.foo = true,
       }
   }
}

fn main() {
    let (s, files) = Settings::default().parse(args_os());
    println!("{:?}", files);
}