@Override
    public List<ExpressionNode> children() {
        ArrayList<ExpressionNode> children = new ArrayList<>();
        children.add(testValue);
        children.addAll(setValues);
        return children;
    }
Returns an immutable list of the values of the set
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/SetMembershipNode.java