presto-main/src/main/java/io/prestosql/sql/planner/planprinter/IoPlanPrinter.java/IoPlanPrinter/processChildren
class IoPlanPrinter:        private Void processChildren(PlanNode node, IoPlanBuilder context)
        {
            for (PlanNode child : node.getSources()) {
                child.accept(this, context);
            }

            return null;
        }