code: func (rt *skipListRT) Add(entries ...rangetree.Entry) rangetree.Entries {
  	overwritten := make(rangetree.Entries, len(entries))
  	for i, e := range entries {
  		overwritten[i] = rt.add(e)
  	}
  
  	return overwritten
  }
docs: // Add will add the provided entries to the tree.  This method
  // returns a list of entries that were overwritten in the order
  // in which entries were received.  If an entry doesn't overwrite
  // anything, a nil will be returned for that entry in the returned
  // slice.