code: private void appendTo(StringBuilder sb) {
      if (sb.length() > 1) {
        sb.append(", ");
      }
      sb.append(key).append(": ").append(value);
      for (HamtPMap<K, V> child : children) {
        child.appendTo(sb);
      }
    }
docs: Appends this map's contents to a string builder.