code: @Override
    @SuppressWarnings("unchecked")
    public void clear() {
      message =
          (MType) (message != null ? message.getDefaultInstanceForType() : builder
              .getDefaultInstanceForType());
      if (builder != null) {
        dispose(builder);
        builder = null;
      }
      onChanged();
    }
docs: Clears the value of the field.
  
  @return the builder