Write article about SRS flash cards

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Reference ​Spaced repetition:

Spaced repetition is an evidence-based learning technique that is usually performed with flashcards. Newly introduced and more difficult flashcards are shown more frequently, while older and less difficult flashcards are shown less frequently in order to exploit the psychological spacing effect. The use of spaced repetition has been proven to increase the rate of learning.

Focus on ​Leitner system:

The Leitner system[1][2][3] is a widely used method of efficiently using flashcards that was proposed by the German science journalist Sebastian Leitner in 1972.[4] It is a simple implementation of the principle of spaced repetition, where cards are reviewed at increasing intervals.