docs: private ClusteringFeature findLeaf(TreeNode node, NumberVector nv) {
      // Find closest child:
      ClusteringFeature[] cfs = node.children;
      assert (cfs[0] != null) : "Unexpected empty node!";
  
      // Find the best child:
      ClusteringFeature best = cfs[0];
      double bestd = distance.squaredDistance(nv, best);
      for(int i = 1; i < cfs.length; i++) {
        ClusteringFeature cf = cfs[i];
        if(cf == null) {
          break;
        }
        double d2 = distance.squaredDistance(nv, cf);
        if(d2 < bestd) {
          best = cf;
          bestd = d2;
        }
      }
  
      return (best instanceof TreeNode) ? findLeaf((TreeNode) best, nv) : best;
    }