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1. Pseudo OOP structure using a dictionary

   /call { exch begin load exec end } def
   
   /undoManager 10 dict def
   
   undoManager begin
       /MAX 3 def
       /ringBuffer MAX array def
       /head 0 def
       /print { ringBuffer == } def
       /do {
           ringBuffer head 3 -1 roll put
           /head head 1 add MAX mod def
       } def
       /reset { /ringBuffer MAX array def } def
       /undo {
           /head head 0 eq { MAX 1 sub } { head 1 sub } ifelse def
           ringBuffer head get % result
           ringBuffer head null put
       } def
   end
   
   3 undoManager /do call
   4 undoManager /do call
   5 undoManager /do call
   6 undoManager /do call
   
   undoManager /print call
   

   [6 4 5]

   undoManager /undo call ==
   

   6

   undoManager /print call
   

   [null 4 5]