presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/EdgePrinter/visitPlan
class EdgePrinter:        @Override
        protected Void visitPlan(PlanNode node, Void context)
        {
            for (PlanNode child : node.getSources()) {
                printEdge(node, child);

                child.accept(this, context);
            }

            return null;
        }