text: // stacker.cpp -- testing the Stack class
  #include <iostream>
  #include <cctype> // or type.h
  #include "stack.h"
  
  int main()
  {
      using namespace std;
      Stack st; // create an empty stack
      char ch;
      unsigned long po;
      cout << "Please enter A to add a purchase order,\n"
           << "P to process a PO, or Q to quit.\n";
      while (cin >> ch && toupper(ch) != 'Q')
      {
          while (cin.get() != '\n')
              continue;
          if (!isalpha(ch))
          {
              cout << "\a";
              continue;
          }
          switch (ch)
          {
          case 'A':
          case 'a':
              cout << "Enter a PO number to add: ";
              cin >> po;
              if (st.isfull())
                  cout << "stack already full\n";
              else
                  st.push(po);
              break;
          case 'P':
          case 'p':
              if (st.isempty())
                  cout << "stack already empty\n";
              else
              {
                  st.pop(po);
                  cout << "PO #" << po << " poped\n";
              }
              break;
          }
          cout << "Please enter A to add a purchase order,\n"
               << "P to process a PO, or Q to quit.\n";
      }
      cout << "Bye\n";
      return 0;
  }