code: public static function emulate($emulation, callable $assertionCallable = null)
      {
          if ((is_string($emulation)) && (class_exists($emulation))) {
              $emulation = new $emulation($assertionCallable);
          }
  
          static::$emulation = $emulation;
      }
docs: Set the emulation to use.
  
  @param string|callable|object $emulation The emulation to use.
  @param callable|null $assertionCallable The assertion call to use.
  @return void