code: public <K, V> EntryStream<K, V> flatMapToEntry(Function<? super T, ? extends Map<K, V>> mapper) {
          return new EntryStream<>(stream().flatMap(e -> {
              Map<K, V> s = mapper.apply(e);
              return s == null ? null : s.entrySet().stream();
          }), context);
      }
docs: Creates a new {@code EntryStream} populated from entries of maps produced
  by supplied mapper function which is applied to the every element of this
  stream.
  
  <p>
  This is an <a href="package-summary.html#StreamOps">intermediate</a>
  operation.
  
  @param <K> the type of {@code Map} keys.
  @param <V> the type of {@code Map} values.
  @param mapper a non-interfering, stateless function to apply to each
  element which produces a {@link Map} of the entries corresponding
  to the single element of the current stream. The mapper function
  may return null or empty {@code Map} if no mapping should
  correspond to some element.
  @return the new {@code EntryStream}