From: fateman@gmail.com   
      
   On Sunday, June 24, 2018 at 1:07:30 PM UTC-7, S.K.R. de Jong wrote:   
   snip   
    However, a straightforward Lisp implementation, similar to    
   > the one above, would choke trying to compute A(4, 4). Hence my question.   
      
   So your question is, does any CAS implement Knuth's "up arrow" notation.   
      
   (a) Not to my knowledge, but it is possible.   
   (b) It appears that it is used in order to return a compact answer from an   
   iterative version of Ackermann's function.   
   (c) It is probably rather low interest otherwise.   
   (d) If you are interested in it, and as you say, know how to use it "by hand"   
   then you should be able to simulate whatever it is you do by hand, by computer   
   program. In that case, it may be useful to have a CAS, because you can   
   manipulate forms like    
   uparrow(a,b).   
      
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