i'm not exactly sure what he's talking about, but there's 2 things Lua has here. one is that the language is functional, so you can replace, alter, or remove the dangerous functions. the other is that you can specify the variable 'environment' of a function with a table (Lua's hash.) so you can construct its environment in an 'opt-in' basis and know for sure everything you're allowing the function to do is safe. and of course these things would be meaningless if built-in functions didn't behave like user functions