[eqvinox]

That's besides my point, which is why I said "structure, not just expression".

It could've used a struct rather than wedging tags into a double's first byte and still be 99 lines.

[fami-com]

That's a standard technique in interpreters. All non-toy Javascript engines use it, for example.

[Spivak]

If that's the trick you object to then you will be sad to hear that Ruby uses it.

[cardiffspaceman]

Nan-boxing is awesome.