def METHOD_NAME(
    self, sequential_index: Optional[int]
) -> Optional[int]:
    index: int = 0
    if sequential_index is not None:
        index_ = self._indices[sequential_index]
        if self._database_list is None:
            assert isinstance(index_, int)
            index = index_
        else:
            assert isinstance(index_, list)
            index = index_[0]
    return index