presto-main/src/main/java/io/prestosql/sql/planner/optimizations/UnaliasSymbolReferences.java/Rewriter/visitTopN
class Rewriter:        @Override
        public PlanNode visitTopN(TopNNode node, RewriteContext<Void> context)
        {
            PlanNode source = context.rewrite(node.getSource());

            SymbolMapper mapper = new SymbolMapper(mapping);
            return mapper.map(node, source, node.getId());
        }