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I would like to make some claims...

Claim 1: We lied: nils are not first-class values (because we cannot
store them in talbes...).

Claim 2: no magic will allow '#' to recognize nils at the "end" of
a table. If we want lists with nils in the end, we not only need
some 'n' field (that is, stored somewhere), but we also need some
way to manipulate this field (t.n=t.n+1, table.setn, something like
this). T[magic]=nil simply does not work.

Claim 3: if we want lists with nils in the middle (holes), we better
have lists with nils in the end. Too often we build lists adding
elements one by one. To build a list with a nil in the middle, somewhere
along the way it must have a nil in the end.

So, changing the definition of '#' will not solve the problem of
lists with nils.

Lists with nils need a somewhat explicit notion of size. We can bury
this 'n' deep inside Lua and create some magic operations to manipulate
them, or we can store it in the clear. In the past we used the famous
".n" field, but there were all kinds of complains against it. Maybe
we could store this size in t[0]? (At least it is a numeric key ;)

-- Roberto