What's the point of LUA_USERSTATE?

["Curt Carpenter" <curtc@...>]

I was trying to see if I can get away with defining LUA_USERSTATE before
including lua.h instead of modifying Lua to store and expose user data
(which I do currently), but since the definition of lua_State isn't
included in lua.h, how are you supposed to access LUA_USERSTATE given a
lua_State?

Thanks,

Curt