... as an l-value to get all results of a function call?
at least ...[i] and #...
extend to full tuples?

__and __or __not metamethods

lua_tostring, lua_tonumber, lua_touseradta etc push values in stack
__tostring,__tonumber, __touserdata metamethods are checked
and expected to push an object of correct type on stack

lua_rawtostring, lua_rawtonumber, lua_rawtouserdata don't
push anything on stack, return data of appropriate type, 
skip metamethods and throw error if object not of exact type

package.findfile exported
module not polluting the global namespace

coxpcall with a coroutine pool for efficiency (reusing coroutines)

exception mechanism formalized? just like the package system was.

a nice bitlib in the core
