Ackermann = WF_Rel + consts ack :: "nat * nat => nat" recdef ack "measure(%m. m) ** measure(%n. n)" "ack(0,n) = Suc n" "ack(Suc m,0) = ack(m, 1)" "ack(Suc m,Suc n) = ack(m,ack(Suc m,n))" end