Object subclass: #Account.
Account instanceVariableNames: 'balance'.

Account class extend [
       new [
           | r |
           <category: 'instance creation'>
           r := super new.
           r init.
           ^r
      ]
   ]

  Account extend [
        init [
            <category: 'initialization'>
            balance := 0
        ]
    ]

  Account extend [
              spend: amount [
                  <category: 'moving money'>
                  balance := balance - amount
              ]
              deposit: amount [
                  <category: 'moving money'>
                  balance := balance + amount
              ]
          ]

  Account extend [
       printOn: stream [
           <category: 'printing'>
           super printOn: stream.
           stream nextPutAll: ' with balance: '.
           balance printOn: stream.
       ]
   ]