A value for "nothing"

[HiPhish]

Hello Schemers,

I am writing an implementation of MessagePack [1] for Guile and a part of the 
spec is the presence of a "nil" data type. What would be a good value to 
express "nothing" in Guile? I cannot use '() because that would be 
indistinguishable from the empty list, so I thought that the return value of a 
function that returns nothing would be a good fit. The function `display` for 
example returns an `#<unspecified>` value, but the only way of producing it 
without side effects so for is the value of `(if #f #f)`. Is there a better 
way?

In Racket there is the `(void)` [2] procedure which returns a `#<void>` 
object, so that's what I am using there [3][4]. Any suggestions for Guile?

[1] https://msgpack.org/
[2] http://docs.racket-lang.org/reference/void.html?q=void
[3] https://gitlab.com/HiPhish/MsgPack.rkt/blob/master/msgpack/unpack.rkt#L47
[4] https://gitlab.com/HiPhish/MsgPack.rkt/blob/master/msgpack/pack.rkt#L35