Derivable Behaviors (Theorems)

• If (n = S.size()) is followed by k push operations then n + k = S.size()
• If (n = S.size()) is followed by k pop operations then n - k = S.size()

  in particular, k <= n
• The last element of S pushed onto S is the top of S
• S.pop() removes the last element of S pushed onto S
• Any two stacks of type T are isomorphic