a > 0 & b > 0 
a < 0 & b < 0 a > 0 & b > 0 a < 0 & b < 0 a > 0 & b > 0 a < 0 & b < 0
b > 0 a > 0 & b > 0 a < 0 & b < 0
i,j:. i < j i = j i > j [a;0]
| 1 | 4. a < 0 a > 0 & b > 0 a < 0 & b < 0 |
| 2 | 4. a = 0 a > 0 & b > 0 a < 0 & b < 0 |
| 3 | 4. a > 0 a > 0 & b > 0 a < 0 & b < 0 |
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