replaced apply-style proof for instance Multiset :: plus_ac0 by recommended Isar proof style
consts ack :: "nat*nat => nat"
recdef ack "measure(%m. m) <*lex*> 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))"