Object extend [ as: similarClass [ ^similarClass newFrom: self ] ]
 
Object subclass: PrimePrice [
	| value |
	value [ ^value ]
	value: newValue [ value := newValue ]
]
 
PrimePrice class extend [ 
	value: value [ ^self new value: value; yourself ] 
]
 
Object subclass: MarginPrice [
	| originalPrice |
	setOriginalPrice: price [ originalPrice := price ]
    value: newValue [ originalPrice value: newValue ]
]
 
MarginPrice class extend [ 
	newFrom: price [ ^self new setOriginalPrice: price; yourself ] 
]
 
MarginPrice subclass: DoublePrice [ value [ ^originalPrice value * 2 ] ]
 
MarginPrice subclass: WholesalePrice [
	| advantage |
	value [ ^originalPrice value + advantage ]
	advantage: newAdvantage [ advantage := newAdvantage ]
]
 
| price |
price := ((((((PrimePrice value: 120)
	as: DoublePrice) 
		as: WholesalePrice) advantage: 80) 
			as: DoublePrice) 
				as: WholesalePrice) advantage: 200.
price value displayNl.
(price value: 100; value) displayNl

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