docs: private void checkCoverTree(Node cur, int[] counts, int depth) {
      counts[0] += 1; // Node count
      counts[1] += depth; // Sum of depth
      counts[2] = depth > counts[2] ? depth : counts[2]; // Max depth
      counts[3] += cur.singletons.size() - 1;
      counts[4] += cur.singletons.size() - (cur.children == null ? 0 : 1);
      if(cur.children != null) {
        ++depth;
        for(Node chi : cur.children) {
          checkCoverTree(chi, counts, depth);
        }
        assert (!cur.children.isEmpty()) : "Empty childs list.";
      }
    }