For when you want to combine more than one stream. For example, the use case that caused us to write this. It shouldn't be any more complex - it would be super minor changes.

```coffee
module.exports = (streams...) ->
  throw new Error 'Expected Readable streams' unless _(streams).all is_readable
  output = new PassThrough objectMode: _(streams).any (stream) -> stream._readableState.objectMode
  cb = _.after streams.length, -> output.end()
  for stream in streams
    stream.on 'end', cb
    stream.pipe output, end: false
  output
```