presto-main/src/main/java/io/prestosql/sql/planner/SubPlan.java/SubPlan/SubPlan
class SubPlan:    public SubPlan(PlanFragment fragment, List<SubPlan> children)
    {
        requireNonNull(fragment, "fragment is null");
        requireNonNull(children, "children is null");

        this.fragment = fragment;
        this.children = ImmutableList.copyOf(children);
    }