title: Waiting for n channels with core.async
body: <p>In the same way <code>alt!</code> waits for <strong>one of</strong> n channels to get a value, I'm looking for the idiomatic way to wait for <strong>all</strong> n channels to get a value.</p>
  
  <p>I need this because I "spawn" n go blocks to work on async tasks, and I want to know when they are all done. I'm sure there is a very beautiful way to achieve this.</p>