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1.◊(∃x)Ux
2. □(x)(Ux→□Ox)
3.assumption: ~(◊∃x)Ox
4. W: (∃x)Ux (from 1)
5. (x)~Ox (from 3)
6. W: Ua (from 4)
7.W: (x)(Ux→□Ox) (from 2)
8. ~Oa (from 5)
9: W: (Ua→□Oa) (from 7)
10: □Oa (from 6 and 9)
11: Oa(from 10)
C: (∃x)Ox (from 3; 8 contradicts 11)