docs: public static <T> Partition<T> singletons(Collection<T> nodes) {
          final Map<T, T> parents = Maps.newHashMap();
          final Map<T, Integer> ranks = Maps.newHashMap();
          for (T node : nodes) {
              parents.put(node, node); // each node is its own head
              ranks.put(node, 0); // every node has depth 0 to start
          }
          return new Partition<T>(parents, ranks);
      }