public function concat()
    {
        $args = func_get_args();
        array_unshift($args, $this->sfa);

        // Concat this iterator, and variadic args
        $class = new ReflectionClass('Qaribou\Iterator\ConcatIterator');
        $concatIt = $class->newInstanceArgs($args);

        // Create as new immutable's iterator
        return new static($concatIt);
    }