"An easy example illustrating this point is maybe the following. Consider the sequence 'MDMD' and suppose that you can only store, say these 4 letters. A 4-letter-strategy might predict something like 'MD' for the next two letters. If those letters though represent the initial of the weekdays, the next 3 letters will be 'FSS'. It is impossible though to predict this sequence solely using information about the past on the last 4 letters. The situation changes if we can store up to 7 letters 'FSSMDMD'. Then a prediction is possible."

I was confused for a while, then I realized the guy is german.

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