Usage of _PROMPT and _PROMPT2

["Soni L." <fakedme@...>]

So I took a quick glance at the lua.c (interpreter) source and found out 
you could do this:

$ lua
Lua 5.3.2  Copyright (C) 1994-2015 Lua.org, PUC-Rio
> _PROMPT="~> "
~> _PROMPT="=> "
=> _PROMPT2="~> "
=> do
~> end
=>

The Lua 5.3 manual says, about names, "As a convention, programs should 
avoid creating names that start with an underscore followed by one or 
more uppercase letters (such as _VERSION).", and about the registry, "As 
with variable names, string keys starting with an underscore followed by 
uppercase letters are reserved for Lua.", and the Lua interpreter/REPL 
is a program. Does this mean it's always safe to use the globals _PROMPT 
and _PROMPT2? Or should the REPL be moved into a preloaded "repl" 
library instead?

Also why isn't this documented anywhere?

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