@Override
    public SetMembershipNode setChildren(List<ExpressionNode> children) {
        if (children.size()<1) throw new IllegalArgumentException("A set membership test must have at least 1 child");
        return new SetMembershipNode(children.get(0), children.subList(1, children.size()));
    }
Returns an immutable list of the values of the set
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/SetMembershipNode.java