code: public String contents()
      {
          StringBuilder builder = new StringBuilder();
          builder.append("{");
          for (Map.Entry<RowPosition, AtomicBTreeColumns> entry : rows.entrySet())
          {
              builder.append(entry.getKey()).append(": ").append(entry.getValue()).append(", ");
          }
          builder.append("}");
          return builder.toString();
      }
docs: for debugging