@ParallelSupported
    public <A, D> Map<K, D> toMap(final Collector<? super Map.Entry<K, V>, A, D> downstream) {
        final Function<Map.Entry<K, V>, K> keyMapper = Fn.key();
        return s.toMap(keyMapper, downstream);
    }
@ ParallelSupported public < A , D > Map < K , D > toMap ( final Collector < ? super Map . Entry < K , V > , A , D > downstream ) { final Function < Map . Entry < K , V > , K > keyMapper = Fn . key ( ) ; return s . toMap ( keyMapper , downstream ) ; }
@param downstream
@return
@see Collectors#groupingBy(Function, Collector)