public String toString() {

    StringBuilder buffer = new StringBuilder("{");

    List<Entry<Integer, FastSparseSet<Integer>>> lst = entryList();
    if (lst != null) {
      boolean first = true;
      for (Entry<Integer, FastSparseSet<Integer>> entry : lst) {
        if (!first) {
          buffer.append(", ");
        }
        else {
          first = false;
        }

        Set<Integer> set = entry.getValue().toPlainSet();
        buffer.append(entry.getKey()).append("={").append(set.toString()).append("}");
      }
    }

    buffer.append("}");
    return buffer.toString();
  }
public String toString()