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nat prime div each factor11a1 1. X :
2. W:. 0<WW<X (t:. X | tWX | t)
3. prime(X)
4. a :
5. b :
6. X | ab 7. 2X 8. x :
9. r : X 10. a = xX+r 11. y :
12. s : X 13. b = yX+s X | aX | b