Object subclass: Book [
    | title borrowed |
 
    Book class >> title: aTitle [
        ^self new initializeTitle: aTitle
    ]
 
    initializeTitle: aTitle [
        title := aTitle.
        borrowed := false.
    ]
 
    borrow [
        borrowed
            ifTrue: [Transcript show: title , ' is already borrowed'; cr.]
            ifFalse: [
                borrowed := true.
                Transcript show: title , ' borrowed'; cr.
            ].
    ]
 
    returnBook [
        borrowed := false.
        Transcript show: title , ' returned'; cr.
    ]
]
 
Object subclass: User [
    | name |
 
    User class >> name: aName [
        ^self new initializeName: aName
    ]
 
    initializeName: aName [
        name := aName.
    ]
 
    borrowBook: aBook [
        Transcript show: name , ' borrows a book'; cr.
        aBook borrow.
    ]
]
 
Object subclass: Library [
    | books |
 
    Library class >> newLibrary [
        ^self new initialize
    ]
 
    initialize [
        books := OrderedCollection new.
    ]
 
    addBook: aBook [
        books add: aBook.
    ]
]
 
| library book user |
 
library := Library newLibrary.
book := Book title: 'Smalltalk Programming'.
user := User name: 'Sato'.
 
library addBook: book.
user borrowBook: book.
book returnBook.

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