docs: private static double findPrototypeSingleton(DistanceQuery<?> dq, DBIDs cx, DBIDRef cy, DBIDVar prototype) {
      double maxDisty = 0., minMaxDist = Double.POSITIVE_INFINITY;
      for(DBIDIter i = cx.iter(); i.valid(); i.advance()) {
        // Maximum distance of i to the element in cy
        double maxDist = dq.distance(i, cy);
        // Maximum of distances from cy.
        maxDisty = (maxDist > maxDisty) ? maxDist : maxDisty;
        if(maxDist >= minMaxDist) {
          // We know a better solution already.
          continue;
        }
        // Maximum distance of i to all other elements in cx
        maxDist = findMax(dq, i, cx, maxDist, minMaxDist);
  
        if(maxDist < minMaxDist) {
          minMaxDist = maxDist;
          prototype.set(i);
        }
      }
      // Singleton point.
      if(maxDisty < minMaxDist) {
        minMaxDist = maxDisty;
        prototype.set(cy);
      }
      return minMaxDist;
    }