code: public <KK> EntryStream<KK, V> mapToKey(BiFunction<? super K, ? super V, ? extends KK> keyMapper) {
          return new EntryStream<>(stream().map(
              e -> new SimpleImmutableEntry<>(keyMapper.apply(e.getKey(), e.getValue()), e.getValue())), context);
      }
docs: Returns an {@code EntryStream} consisting of the entries whose keys are
  modified by applying the given function and values are left unchanged.
  
  <p>
  This is an <a href="package-summary.html#StreamOps">intermediate</a>
  operation.
  
  @param <KK> The type of the keys of the new stream
  @param keyMapper a non-interfering, stateless function to apply to each
  key-value pair which returns the updated key
  @return the new stream
  @since 0.3.0