nil is just an atom, specialcased by the parser to be able to leave off the colon.
In my opinion having marker or value is deemed to fail anyway. What if is I want to insert exactly the value you use as marker?
Please use a tagged tuple or at least provide an API similar to Map get/fetch/fetch! to be able to differentiate when necessary.
Also when you say, it’s collectable and enumerable, in which direction? Is there a way to collect/enumerate in the opposite direction?
