presto-main/src/main/java/io/prestosql/sql/planner/optimizations/joins/JoinGraph.java/JoinGraph/visitGroupReference
class JoinGraph:        @Override
        public JoinGraph visitGroupReference(GroupReference node, Context context)
        {
            PlanNode dereferenced = lookup.resolve(node);
            JoinGraph graph = dereferenced.accept(this, context);
            if (graph.nodes.size() == 1) {
                return replacementGraph(getOnlyElement(graph.nodes), node, context);
            }
            return graph;
        }