You should think about how "the middle one" is different from the others and you should think about other data records and what you want done with them. What results would you want with the following lines?

+ 1 + 2 + 3 +
+ 1 + 2 + 3 + 4 +
+1+2+3+
+1+2+3+4+
++1+2++
1 + 2
1 + 2 + 3
1 + 2 + 3 + 4
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