bottom-up implementation

bottom-up implementation n.  Hackish opposite of the
   techspeak term `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.