function newTable(t)
   if not t then
      t = {}
   end
   setmetatable(t, {__index = table})
   return t
end

t = newTable({k1="v1", k2="v2", k3="v3",})


function table:keys()
   k = newTable()
   for key in pairs(self) do
      k:insert(key)
   end
   return k
end

print(t:keys():concat(" "))