presto-main/src/main/java/io/prestosql/sql/planner/optimizations/SetFlatteningOptimizer.java/SetFlatteningOptimizer/visitPlan
class SetFlatteningOptimizer:        @Override
        public PlanNode visitPlan(PlanNode node, RewriteContext<Boolean> context)
        {
            return context.defaultRewrite(node, false);
        }