code: static private function assertValidChangeSet(array $cs)
      {
          if (
              array_key_exists(0, $cs) &&
              array_key_exists(1, $cs) &&
              (is_null($cs[0]) || static::isValidState($cs[0])) &&
              (is_null($cs[1]) || static::isValidState($cs[1]))
          ) {
              return true;
          }
  
          throw new InvalidArgumentException("Unexpected order state change set.");
      }
docs: Returns whether or not the change set is valid.
  
  @param array $cs
  
  @return bool
  
  @throws InvalidArgumentException