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: crossfade.rs
    path of file: ./repos/rodio/src/source
    
    the code of the file until where you have to start completion: use std::time::Duration;

use cpal::FromSample;

use crate::source::{FadeIn, Mix, TakeDuration};
use cra
    
    fn test_crossfade() {
        let source1 = dummysource(10);
        let source2 = dummysource(10);
        let mut mixed = crossfade(
            source1,
            source2,
            Duration::from_secs(5) + Duration::from_nanos(1),
        );
        assert_eq!(mixed.next(), Some(1.0));
        assert_eq!(mixed.next(), Some(2.0));
        assert_eq!(mixed.next(), Some(3.0));
        assert_eq!(mixed.next(), Some(4.0));
        assert_eq!(mixed.next(), Some(5.0));
        assert_eq!(mixed.next(), None);

        let source1 = dummysource(10);
        let source2 = dummysource(10).amplify(0.0);
        let mut mixed = crossfade(
            source1,
            source2,
            Duration::from_secs(5) + Duration::from_nanos(1),
        );
        assert_eq!(mixed.next(), Some(1.0 * 1.0));
        assert_eq!(mixed.next(), Some(2.0 * 0.8));
        assert_eq!(mixed.next(), Some(3.0 * 0.6));
        assert_eq!(mixed.next(), Some(4.0 * 0.4));
        assert_eq!(mixed.next(), Some(5.0 * 0.2));
        assert_eq!(mixed.next(), None);
    }