private void popStack(Deque<ValueItem> stack) {
        ValueItem rhs = stack.pop();
        ValueItem lhs = stack.peek();
        lhs.value = rhs.op.evaluate(lhs.value, rhs.value);
    }
Returns true if this node has lower precedence than the parent
(even though by virtue of being a node it will be calculated before the parent).
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/OperationNode.java