Prevent table insertions?

["Jason Perkins" <starkos@...>]

I'm looking to find a way to prevent new values from being entered
into a table. I tried the read-only table approach from PIL
(http://www.lua.org/pil/13.4.5.html) which works as advertised.
However, I can still insert values like:

 table.insert(days, "Frankday")

Any suggestions?

Thanks!
Jason