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consts ack :: "nat*nat => nat"
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recdef ack "measure(%m. m) <*lex*> measure(%n. n)"
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"ack(0,n) = Suc n"
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"ack(Suc m,0) = ack(m, 1)"
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"ack(Suc m,Suc n) = ack(m,ack(Suc m,n))"