Choice between non-flat documents

How do I construct a document that renders as

```
abcd
efgh
```

when the page is wide enough, or alternatively as

```
ab
cd
ef
gh
```

?

Using the public API I would try this:

```haskell
x = "abcd" <> hardline <> "efgh"
y = concatWith (\a b -> a <> hardline <> b) ["ab", "cd", "ef", "gh"]
doc = group (flatAlt y x)
```

But this will always render `y`:

```
> putDoc doc
ab
cd
ef
gh
```

…because `group` attempts and fails to flatten the preferred version:

https://github.com/quchen/prettyprinter/blob/57ced7bf35ef581a247d79a4a1b803b384845cc4/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs#L580-L585

https://github.com/quchen/prettyprinter/blob/57ced7bf35ef581a247d79a4a1b803b384845cc4/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs#L627-L628

https://github.com/quchen/prettyprinter/blob/57ced7bf35ef581a247d79a4a1b803b384845cc4/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs#L653-L657

This is an interesting difference to `ansi-wl-pprint` which [doesn't](https://github.com/ekmett/ansi-wl-pprint/blob/1d56efff10d30810ac58779f3e7edfa573f9add8/Text/PrettyPrint/ANSI/Leijen/Internal.hs#L727-L753) attempt to flatten the "nice" `FlatAlt` branch.

If I resort to the internals, it's easy:

```haskell
doc' = Union x y
```

```
> putDocW 4 doc'
abcd
efgh
```

```
> putDocW 2 doc'
ab
cd
ef
gh
```

Is there a different way to achieve this with the public API? Is this simply a problem that doesn't arise in the "real world"?