public String toString(Collection<?> collection) {
        StringBuilder buffer=new StringBuilder();
        for (Iterator<?> i=collection.iterator(); i.hasNext(); ) {
            buffer.append(i.next().toString());
            if (i.hasNext())
                buffer.append(", ");
        }
        return buffer.toString();
    }
Returns an modifiable view of the destination set owned by this
config-model/src/main/java/com/yahoo/vespa/documentmodel/SummaryField.java