The opposite of top-down design. It is now received wisdom in most programming cultures that it is best to design from higher levels of abstraction down to lower, specifying sequences of action in increasing detail until you get to actual code. Hackers often find (especially in exploratory designs that cannot be closely specified in advance) that it works best to *build* things in the opposite order, by writing and testing a clean set of primitive operations and then knitting them together.

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