From: julio@diegidio.name   
      
   On Monday, 4 April 2022 at 15:46:12 UTC+2, Malcolm McLean wrote:   
   > On Monday, 4 April 2022 at 14:02:24 UTC+1, ju...@diegidio.name wrote:   
   > >   
   > > In fact, a stack that does not implement access by index just wouldn't cut   
   it.   
   > >   
   > You can only undo the item on top of the stack.   
      
   At that point I had explained exactly how to do it with one stack: indeed you   
   just need a stack pointer and what you can do with that.   
      
   > If you have two stacks, they can be true stacks   
      
   There is no such thing as a "true stack", there is the basic stack from   
   programming course #1 and then there is more.   
      
   Julio   
      
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