code: public function transformAndConcat(callable $transformer) : FluentIterable
      {
          return self::from(Iterables::concatIterables($this->transform($transformer)));
      }
docs: Applies function to each element of this fluent iterable and returns a fluent iterable
  with the concatenated combination of results. Transformer returns a Traversable of results.
  
  @param callable $transformer
  @return FluentIterable
  @see flatMap