30 years of SuperMemo apps

30 years ago yesterday, SuperMemo 1.0 for DOS was born. Soon it had its first users, mostly co-founders of SuperMemo World. Four years later it was turned into a business. It needed another 14 years before the birth of the copycat industry. Today, spaced repetition is a must for every respectable learning application. Most of them rely on SuperMemo World publications and research.