let s = "";
println!("is empty: {}", s.is_empty());
let t = "x";
println!("is empty: {}", t.is_empty());
let a = String::new();
println!("is empty: {}", a.is_empty());
let b = "x".to_string();
println!("is empty: {}", b.is_empty());
println!("is not empty: {}", !b.is_empty());