@Override
    public NegativeNode setChildren(List<ExpressionNode> children) {
        if (children.size() != 1) throw new IllegalArgumentException("Expected 1 children but got " + children.size());
        return new NegativeNode(children.get(0));
    }
Returns the node creating the value negated by this
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/NegativeNode.java