main_4.rs

// Cloning merge_chan first, then moving it. 
// This move ensures that when square_workers drop,
// and the workers quit their loop,
// all merge_chan senders drop as well, 
// triggerign the quit of the loop in the "merge" stage...
let mut square_workers: VecDeque<Sender<PipelineMsg>> = vec![square(merge_chan.clone()),
                                                             square(merge_chan)]
                                                             .into_iter()
                                                             .collect();