You can add `infixl` or `infixr` to that:

    let x = x in do case if let infixr 7 `x`; x = x in True then False else False of _ -> return False

And with the RecursiveDo extension, you can throw `mdo` in there:

    let x = x in mdo do case if let infixr 7 `x`; x = x in True then False else False of _ -> return False

That brings us to `in mdo do case if let infixr` as the longest string of consecutive (distinct) keywords that I can come up with in Haskell.