Contain contents not parents

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A class must know what it contains, but it should never know who contains it. --Riel's Heuristic 4.13, Arthur Riel 1996

This heuristic refers to the idea that classes should not know what objects contain them as this makes them far less reusable. If a class is designed for a specific use and is dependent on a parent object, it can't then be used inside a different type of parent object. For example, Riel uses the example of a BedRoom containing an AlarmClock. The BedRoom object must know about the AlarmClock object inside it, but if AlarmClock is dependent on BedRoom then it can't then be used inside a TimeLockSafe.

The Chain of Responsibility design pattern violates this heuristic.