Why OP_CLOSE has been removed?

[Marco Bambini <marco@...>]

Hi all,
I was studying the "Closures in Lua" PDF document published some time ago.
The document explains the usage of the CLOSE opcode (OP_CLOSE) which has been removed in recents Lua versions.

I wondering why this opcode has been removed.
It is now always used implicitly? (I guess in RETURN and JUMP but what about when a var goes out scope in a block?)
I think that this choice could add a lot of overhead, am I wrong?

Thanks a lot for the explanation.
--
Marco Bambini