code: function empty(): self {
      if (is_object($this->target) && !($this->target instanceof \Countable)) {
        $constraint = countOf(0);
        $target = get_object_vars($this->target);
      }
      else if (is_string($this->target)) {
        $constraint = equalTo(0);
        $target = $this->hasFlag('file') ? @filesize($this->target) : mb_strlen($this->target);
        // TODO file/directory flag handling!
      }
      else {
        $constraint = isEmpty();
        $target = $this->target;
      }
  
      return $this->expect($target, $constraint);
    }
docs: Reports an error if the target is not empty.
  For arrays, strings, and `Countable` instances, it checks the length. For objects, it gets the count of accessible properties according to scope.
  @return self This instance.