public static <E,M,A>ProducerT<E,M,A> done(A r){
  return ProducerT.producerT(FreeT.done(r));
}

public static < E , M , A > ProducerT < E , M , A > done ( A r ) { return ProducerT . producerT ( FreeT . done ( r ) ) ; }
a generator that has finished .
