Docs say you can assoc nil, don't warn that ilookup never returns nil values

[favila]

You can assoc nil onto an entity map; however this has the effect of making that attribute permanently not-found, which is not the impression the docs give.

;; You can assoc any value you want, even types not supported by Datomic
;; such as nil.

If the keyword shadows an ident, the attribute is forever regarded as not-present and will never fall back to db or derived-attr lookup and there's no way to unshadow it.