Lua Basics: modules()?

[William Trenker <wtrenker@...>]

>From studying various Lua modules I see that experienced Lua
programmers usually set up a number of local variables that carry the
value of standard library functions, like this:

local assert, error, _G, loadstring, loadfile, _pcall, type, unpack,
xpcall = assert, error, _G, loadstring, loadfile, pcall, type, unpack,
xpcall

What is the point of this?  On the surface it appears to be redundant.
 But since it is often used by the experts I'm obviously missing
something important.

Thanks very much,
Bill