How to tell if running script directly?

[<tonyp@...>]

Is there an equivalent for Python's

if __name__ == "__main__":
   main()

in Lua?

For example, something like this?  (If not, is this something that could be 
added in the next release?)

if _MAIN then
 ...
end

where _MAIN would be true if the script is run directly, and false if it is 
part of require or dofile.

TIA
Tony.