presto-main/src/main/java/io/prestosql/execution/scheduler/PhasedExecutionSchedule.java/Visitor/Visitor
class Visitor:        public Visitor(Collection<PlanFragment> fragments, DirectedGraph<PlanFragmentId, DefaultEdge> graph)
        {
            this.fragments = fragments.stream()
                    .collect(toImmutableMap(PlanFragment::getId, identity()));
            this.graph = graph;
        }