public static <T,K,U>Collector<T,?,ImmutableMap<K,U>> toImmutableMap(Function<? super T,? extends K> keyMapper,Function<? super T,? extends U> valueMapper){
  return Collector.of(null,null,throwingMerger(),null);
}

public static < T , K , U > Collector < T , ? , ImmutableMap < K , U > > toImmutableMap ( Function < ? super T , ? extends K > keyMapper , Function < ? super T , ? extends U > valueMapper ) { return Collector . of ( null , null , throwingMerger ( ) , null ) ; }
keymapper result must be unique
