code: @Deprecated
      @Beta
      @SequentialOnly
      public <KK, VV> EntryStream<KK, VV> mapER(final Function<? super Map.Entry<K, V>, KK> keyMapper, final Function<? super Map.Entry<K, V>, VV> valueMapper) {
          checkState(s.isParallel() == false, "mapER can't be applied to parallel stream");
  
          final Function<Map.Entry<K, V>, Map.Entry<KK, VV>> mapper = new Function<Map.Entry<K, V>, Map.Entry<KK, VV>>() {
              private final ReusableEntry<KK, VV> entry = new ReusableEntry<>();
  
              @Override
              public Entry<KK, VV> apply(Entry<K, V> t) {
                  entry.set(keyMapper.apply(t), valueMapper.apply(t));
  
                  return entry;
              }
          };
  
          return map(mapper);
      }
docs: To reduce the memory footprint, Only one instance of <code>Map.Entry</code> is created,
  and the same entry instance is returned and set with different keys/values during iteration of the returned stream.
  The elements only can be retrieved one by one, can't be modified or saved.
  The returned Stream doesn't support the operations which require two or more elements at the same time: (e.g. sort/distinct/pairMap/slidingMap/sliding/split/toList/toSet/...).
  , and can't be parallel stream.
  Operations: filter/map/toMap/groupBy/groupTo/... are supported.
  
  <br />
  <code>ER</code> = <code>Entry Reusable</code>
  
  
  @param keyMapper
  @param valueMapper
  @return
  @deprecated