code: @SuppressWarnings("unchecked") // safe covariant cast
    @GwtIncompatible("To be supported")
    <K extends K0, V extends V0> RemovalListener<K, V> getRemovalListener() {
      return (RemovalListener<K, V>) MoreObjects.firstNonNull(removalListener, NullListener.INSTANCE);
    }
docs: /*
  Note that MapMaker's removalListener() is not here, because once you're interacting with a
  GenericMapMaker you've already called that, and shouldn't be calling it again.