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<DATA>; ... 1/2/2007, 1, 4, 5, 6 1/3/2007, 2, 5, 7, 10 1/5/2007, 5, 6, 8, 11
2-1, 5-4, 7-5, 10-6 5-2, 6-5, 8-7, 11-10
<DATA>; ... } @prev_line = @this_line; }