"If you're doing top-down design, you produce a specification that stops at some level of granularity. And you always risk discovering, come implementation time, that the module or class that was the lowest level of your specification hides untold worlds of complexity that will take as much development effort as you'd budgeted for the rest of the project combined. The only way to avoid that is to have your design go all the way down to specifying individual lines of code, in which case you aren't designing at all, you're just programming."

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