-- like map, but support nils inside arr
-- arr must provide its length in an 'n' field,
-- as returned by 'unpack'.
map_nil = function(arr, f)
	local result = {}
	for i=1,arr.n do
		result[i] = f(arr[i])
	end
	result.n = arr.n
	return result
end

-- like table.pack from Lua 5.2
-- https://stackoverflow.com/questions/7183998/in-lua-what-is-the-right-way-to-handle-varargs-which-contains-nil/7186820#7186820
pack = function(...)
	local result = {...}
	result.n = select('#', ...)
	return result
end

	local line = table.concat(map({...}, tostring), ' ')

	local args = pack(...)
	local line = table.concat(map_nil(args, tostring), ' ')