docs: public void bulkLoad(DBIDs ids) {
      if(ids.size() == 0) {
        return;
      }
      assert (root == null) : "Tree already initialized.";
      DBIDIter it = ids.iter();
      DBID first = DBIDUtil.deref(it);
      // Compute distances to all neighbors:
      ModifiableDoubleDBIDList candidates = DBIDUtil.newDistanceDBIDList(ids.size() - 1);
      for(it.advance(); it.valid(); it.advance()) {
        candidates.add(distance(first, it), it);
      }
      root = bulkConstruct(first, Integer.MAX_VALUE, candidates);
    }