“A subsystem is a set of classes (and possibly other subsystems) collaborating to fulfill a set of responsibilities. Although subsystems do not exist as the software executes, they are useful conceptual entities.”

—  Rebecca Wirfs-Brock

Source: Object-oriented design: a responsibility-driven approach (1989), p. 30

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Rebecca Wirfs-Brock 13
American software engineer 1953

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Rebecca Wirfs-Brock (1953) American software engineer

Rebecca Wirfs-Brock (2003) in " An Interview with Rebecca Wirfs-Brock Author of Object Design http://www.objectsbydesign.com/books/RebeccaWirfs-Brock.html" 2003-2005 Objects by Design, Inc: Answer to the question Can you clarify what you consider to be the essential elements of a "conceptual view".

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Page 182 (italics in source).
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