x4. 0
y
x = y 
x = 
yx4. y < 0
x = -y x = y4. 0
y -x = y x = y4. y < 0
-x = -y x = y
if 0
x
x else -x fi = if 0y y else -y fi x = yx)
y)
| 1 | 3. 0x 4. 0 y x = y x = y |
| 2 | 3. 0x 4. y < 0 x = -y x = y |
| 3 | 3. x < 0 4. 0 y -x = y x = y |
| 4 | 3. x < 0 4. y < 0 -x = -y x = y |
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