presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/GraphvizPrinter/visitEnforceSingleRow
class GraphvizPrinter:        @Override
        public Void visitEnforceSingleRow(EnforceSingleRowNode node, Void context)
        {
            printNode(node, "Scalar", NODE_COLORS.get(NodeType.PROJECT));
            return node.getSource().accept(this, context);
        }