lua interpreter to return a value to shell

[Grizzly Bear <6grizzlybear@...>]

My script, a.lua, starts with 

      #!/usr/local/bin/lua

as the first line, i.e., using the interpreter coming with lua-5.1.5,

how can I make the script return a value to indicate success or failure?

That is, I'd like to do under bash

     $ if ./a.lua; then echo "success"; fi

Thanks.

Wei