@SafeVarargs
        public final Rule treeRule(List<Rule>... rules) {
            List<Rule> out = new ArrayList<>();
            for (List<Rule> ruleList : rules) {
                out.addAll(ruleList);
            }
            return this.onBuild.apply(new TreeRule(this, out));
        }
A builder used to create a {@link Rule} class.
smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/syntax/rule/Rule.java