code: public function generate(Finder $finder)
      {
          foreach ($finder as $file) {
              $mutable = new Mutable($file->getRealpath());
  
              $this->mutables[] = $mutable;
          }
      }
docs: Given a file iterator pass each to a \Humbug\Mutable
  instance which is used to generate mutations and store
  the instructions for applying and reversing them as
  a set of mutables (instances of \Humbug\Mutation).
  
  @return void