We present the derivation of a solution to a LISP programming exercise. The derivation is in three steps. First, an inefficient solution is given. Second, the quintessence of a more efficient solution is captured in a number of equalities. Third, an efficient solution is derived from the inefficient one by a number of transformation steps, each of which is justified by the equalities.

© 1992 California Institute of Technology. The above derivation was prompted by questions from my CS20 students and I welcome the opportunity to express my gratitude to them. Comments by H. Peter Hofstee, Johan J. Lukkien, the Austin Tuesday Afternoon Club, and especially the referees improved the presentation.

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