local gbl_mt = {}
local decl = {}

function gbl_mt:__index(k)
	if not decl[k] then
		error("unknown global: "..k, 2)
	end
	return rawget(self, k)
end

function gbl_mt:__newindex(k, v)
	if not decl[k] then
		error("unknown global: "..k, 2)
	end
	rawset(self, k, v)
end

function global(k, v)
	decl[k] = true
	rawset(_G, k, v)
end

setmetatable(_G, gbl_mt)