code: public function add($name, GeneratorInterface $class)
      {
          if (array_key_exists($name, $this->generators)) {
              throw new \InvalidArgumentException('Generator already exists.');
          }
  
          $this->generators[$name] = $class;
      }
docs: Add a generator to the factory
  
  @param string               $name   the name of the Generator
  @param GeneratorInterface   $class  the generator object to return for the specified name