[ParetoOptimal]

> The library didn't allow you to see the things (e.g. particular headers or options for those headers) that it didn't know to parse.

I typically design code around things like this with a sum type like:

    data Header = KnownHeader1 | KnownHeader2 | UnknownHeader String String

Then I typically don't offer any extra support or extended functionality for the cases where the type is `UnknownHeader`.