How can I bind an external variable to a sequence?

I see the syntax trick for escaping commas, but I still don't get a bound sequence.

I tried this:

import org.basex.core.*;
import org.basex.core.cmd.*;
import org.junit.*;

public class BindingTest {

String var = "$ids=";
String seq = "('a',,'b',,'c')";
String query = ""
      + "declare variable $ids external; "
      + "let $count := count($ids) "
      + "return if($count gt 1) then element ids { $ids } "
      + "else error(QName('urn:err', 'not_enough_ids'), xs:string($count)) "
      ;

/**
   * test binding external variable to sequence
   */
@Test
public void test() {
    try {
      Context context = new Context();
      new Set(Prop.BINDINGS, var + seq).execute(context);
      String result = new XQuery(query).execute(context);
      context.close();
      System.out.println(result);
    } catch(Exception e) {
      System.err.println(e);
    }
}

}

I expect to get <ids>...</ids> but instead I get the error. I'm using v7.5, Java 7u13.