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