Sequencing Controllers

I wanted to sequence 2 controllers so that the second ran after the first was completely drained (to be able to send finalizer signals down the shute etc). I found myself digging into the core library to do it, by tweaking the Input <|>

```
sequenced :: Controller a -> Controller a -> Controller a
sequenced (AsInput c0) (AsInput c1) = AsInput (Input $ do
r0 <- recv c0
case r0 of
Nothing -> recv c1
Just a -> return (Just a))
```

And this worked pretty well with applicative style:

```
test1 = runMVC () (asPipe cat)
((,) <$>
pure (contramap show MVC.stdoutLines) <*>
(sequenced <$>
MVC.producer Single (each ['a'..'d']) <*>
MVC.producer Single (each ['1'..'4'])))
```

Is there any other way to sequence Controllers? Or, alternatively, is there a way to lift the hood of the library a little whilst still preventing Controllers from being consumed elsewhere (which I fully appreciate and support)?