rafa/docs/Stream-once.md

Rafa.js is a Javascript framework for building concurrent applications.

Convert the first upstream message to a done message, then emit to children and 
detach from the parent stream.

<aside>
```js
// once(): Stream
var stream = Rafa.stream();
var values = [];

stream.once().done(v => values.push(v));

stream.write(1).write(2);

// values: [1]
```
</aside>