private void combineValues(Deque<ValueItem> buf) {
        ValueItem rhs = buf.pop();
        ValueItem lhs = buf.pop();
        buf.push(new LazyCombinedItem(lhs, rhs));
    }
Combines the top two items of the given stack using the operator of the second.

@param buf The stack of values.
document/src/main/java/com/yahoo/document/select/rule/LogicNode.java