code: function readable(): self {
      if (!$this->hasFlag('directory') && !$this->hasFlag('file'))
        throw new \BadMethodCallException('This assertion is not a file or directory one.');
  
      return $this->expect($this->target, isReadable());
    }
docs: Reports an error if the file or directory specified by the target is not readable.
  @return self This instance.
  @throws \BadMethodCallException This assertion is not a file or directory one.