public LogicNode add(String operator, ExpressionNode node) {
        items.add(new LogicNode.NodeItem(stringToOperator(operator), node));
        return this;
    }
Adds an (operator, node) pair to this expression.

@param operator The operator that combines the previous with the node given.
@param node The node to add to this.
@return This, to allow chaining.
document/src/main/java/com/yahoo/document/select/rule/LogicNode.java