code: public AddAction append(Number... values) {
          return append(new LinkedHashSet<Number>(Arrays.asList(values)));
      }
docs: Returns an <code>AddAction</code> for building expression that would
  append the specified values to this number set; or if the attribute does
  not already exist, adding the new attribute and the value(s) to the item.
  <p>
  In general, DynamoDB recommends using SET rather than ADD.