function print_table(foo, bar)
   for k, v in pairs(foo) do
      print(bar .. k, v)

      if type(v) == 'table' and bar .. k ~= '_G' then
	 print_table(v, bar .. k .. '.')
      end
   end
end

print_table(_G, '')

print(getmetatable('').__index)
print(string)