require explicit variable assign before usage on specified table

setexplicit.lua

function setexplicit(table)
    table = table or _G
    setmetatable(table, {
        declared = {},
        __newindex = function(t, n, v)
            getmetatable(t).declared[n] = true
            rawset(t, n, v)
        end,
        __index = function(t, n)
            if getmetatable(t).declared[n] then
                return nil
            else
                error("undefined variable: "..n, 2)
            end
        end
    })
end