docs: def insert(self, index, filename):
          """Insert a new subclass with filename at index, mockup __module__."""
          base = self._base
          dct = {'__module__': base.__module__, 'filename': filename, '_stack': self}
          cls = type(base.__name__, (base,), dct)
  
          self._map[cls.filename] = cls
          self._classes.insert(index, cls)