private boolean isHeadOf(Iterator<?> full, Iterator<?> head) {
        while (head.hasNext()) {
            if (!full.hasNext()) return false;

            if (!full.next().equals(head.next())) return false;
        }
        return true;
    }
Returns true if the second list is the start of the first list
config-model/src/main/java/com/yahoo/vespa/documentmodel/SummaryField.java