fn main() {
  let search_term = "picture";
  let quote = "Every face, every shop, bedroom window, public-house, and
dark square is a picture feverishly turned--in search of what?
It is the same with books. What do we seek through millions of pages?";
  let mut line_num: usize = 1; // <1>

  for line in quote.lines() {
    if line.contains(search_term) {
      println!("{}: {}", line_num, line); // <2>
    }
    line_num += 1; // <3>
  }
}