code: <C> Converter<A, C> doAndThen(Converter<B, C> secondConverter) {
  		return new ConverterComposition<A, B, C>(this, checkNotNull(secondConverter));
  	}
docs: Package-private non-final implementation of andThen() so only we can
  override it.
  
  @param <C>
  the generic type
  @param secondConverter
  the second converter
  @return the converter