presto-main/src/main/java/io/prestosql/sql/planner/SimplePlanVisitor.java/SimplePlanVisitor/visitPlan
class SimplePlanVisitor:    @Override
    protected Void visitPlan(PlanNode node, C context)
    {
        for (PlanNode source : node.getSources()) {
            source.accept(this, context);
        }
        return null;
    }