@Override
    public String toString() {
        StringBuilder ret = new StringBuilder();
        for (LogicNode.NodeItem item : items) {
            if (item.operator != NOP) {
                ret.append(" ").append(operatorToString(item.operator)).append(" ");
            }
            ret.append(item.node);
        }
        return ret.toString();
    }
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