public AddAction append(Set<String> values) {
        return new AddAction(this, new LiteralOperand(values));
    }
Returns an <code>AddAction</code> for building expression that would
append the specified values to this string set; or if the attribute does
not already exist, add the new attribute and the value(s) to the item.
<p>
In general, DynamoDB recommends using SET rather than ADD.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/xspec/SS.java