code: public static function transform(IteratorAggregate $fromIterable, callable $transformer) : IteratorAggregate
      {
          return new CallableIterable(
              function () use ($fromIterable, $transformer) {
                  return Iterators::transform(Iterators::from($fromIterable->getIterator()), $transformer);
              }
          );
      }
docs: Returns an iterable that applies function to each element of fromIterable.
  
  @param IteratorAggregate $fromIterable
  @param callable $transformer
  @return IteratorAggregate