Reverse Polish Notation: stack after every token
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tokens: [2, 1, +, 3, *]   result = 9
  token 2: push 2   stack(top first) = [2]
  token 1: push 1   stack(top first) = [1, 2]
  token +: pop 1 then 2, compute 2 + 1 = 3   stack(top first) = [3]
  token 3: push 3   stack(top first) = [3, 3]
  token *: pop 3 then 3, compute 3 * 3 = 9   stack(top first) = [9]

tokens: [4, 13, 5, /, +]   result = 6
  token 4: push 4   stack(top first) = [4]
  token 13: push 13   stack(top first) = [13, 4]
  token 5: push 5   stack(top first) = [5, 13, 4]
  token /: pop 5 then 13, compute 13 / 5 = 2   stack(top first) = [2, 4]
  token +: pop 2 then 4, compute 4 + 2 = 6   stack(top first) = [6]