code: @Override
      public boolean validateLabel(String label) {
          Objects.requireNonNull(label, "label cannot be null");
          // check label is in set
          return !labels.contains(label);
      }
docs: Checks the given label can be added/removed to/from a vertex.
  
  @param label The label to validate.
  @return <code>true</code> if the label can be assigned to a vertex, otherwise <code>false</code>.