presto-main/src/main/java/io/prestosql/sql/planner/plan/AssignUniqueId.java/AssignUniqueId/replaceChildren
class AssignUniqueId:    @Override
    public PlanNode replaceChildren(List<PlanNode> newChildren)
    {
        checkArgument(newChildren.size() == 1, "expected newChildren to contain 1 node");
        return new AssignUniqueId(getId(), Iterables.getOnlyElement(newChildren), idColumn);
    }