fn main() {
let xs=vec![1_i64,2,3,4];
trait Describe { fn describe(&self) -> String; }
impl Describe for i64 {
    fn describe(&self) -> String { format!("int {self}") }
}
impl Describe for f64 {
    fn describe(&self) -> String { format!("float {self}") }
}
impl Describe for [i64] {
    fn describe(&self) -> String { format!("{} ints, first {}", self.len(), self[0]) }
}
fn describe<T: Describe + ?Sized>(x: &T) -> String { x.describe() }
println!("{}; {}; {}; {}", describe(&7_i64), 2.5_f64.describe(),
         xs.describe(), describe(&xs[2..]));


}