Some types arrive with their operations already specified — by mathematics. If a type is a number, a set, or a sequence, math has written the complete API for us; the work is to implement it, and to implement it efficiently.

General code comes out better than code cut to fit one job, and its authors are the first to benefit. When we cannot afford all of it, the way down runs against instinct: work out the ideal shape first, then cut what today does not need, and keep a plan for putting it back.