% From The Art of Eyepl, Chapter 24 — Optimization is search plus an order.
best_plan(Request, Plan, Cost) :-
  aggregate_min(
    [CandidateCost, CandidatePlan],
    CandidatePlan,
    candidate_plan(Request, CandidatePlan, CandidateCost),
    [Cost, Plan],
    Plan
  ).