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> But nil isn't really first-class, because it cannot really be stored
> as value into tables.

You can definitely store nil in a table in the sense that t[1]=nil gives
you back nil when you read t[1].

> "storing" it in a table makes the key go away

This reflects our view of tables, which are containers for values.
Keys play a secondary role. When there is no value, there's no key.
This behavior is *not* due to some flaw in nil.

> not show up on i/pairs

One way to think about a table is that it initially contains *all* pairs
(key,nil), for all possible keys. You wouldn't want pairs to show them all :-)