6. x2: T2
7. y1: T1
8. y2: T2
< x1,x2 > = < y1,y2 > 
< x1,x2 > = < y1,y2 > 
x,y:T1. Dec(x = y)
x,y:T2. Dec(x = y)
T2
T2
Dec(x = y)
| 1 | 5. x1: T1 6. x2: T2 7. y1: T1 8. y2: T2 < x1,x2 > = < y1,y2 > < x1,x2 > = < y1,y2 > |
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