docs: public final void clearExtraTag(final String name) {
      if (extratags == null) {
        throw new IllegalStateException("no extra tags added");
      }
      if (extratags.get(name) == null) {
        throw new IllegalArgumentException("tag '" + name
            + "' not in" + extratags);
      }
      extratags.remove(name);
    }