function only_numerics(s)
	local result = ""
	if s ~= nil then
		for c in s:gmatch("%d") do
			result = result .. c
		end
	end
	return result
end

UpdateChecker.read_local_checksum = function()
	local fn = UpdateChecker.WORK_DIR .. UpdateChecker.CHECK_FILENAME
	print("local checksum: " .. fn)
	local s = slurp(fn)
	print("local checksum(text): " .. s)
	s = only_numerics(s)
	local n = tonumber(s)
	print("local checksum(num):" .. tostring(n))
	return n
end