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);
}