Because the requirement for having an interpretation for each line is necessary in the event of it being a changing line. If the third line is changing in one reading, it may not be the next time. If no interpretation exists, how will you read it if the changing line in your next reading is the first line? And in fact, you could have a reading with all lines being changing, but there are rules for determining which changing line prevails. And of course, there is the reading with no changing lines at all, so only the general interpretation is applicable.

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