presto-main/src/main/java/io/prestosql/sql/planner/plan/EnforceSingleRowNode.java/EnforceSingleRowNode/replaceChildren
class EnforceSingleRowNode:    @Override
    public PlanNode replaceChildren(List<PlanNode> newChildren)
    {
        return new EnforceSingleRowNode(getId(), Iterables.getOnlyElement(newChildren));
    }