def make_splitter(delimiter):
    """Create a reducer with a custom delimiter.

    Parameters
    ----------
    delimiter : str
        Delimiter to use to split keys.

    Returns
    -------
    f : Callable
        Callable that can be passed to ``unflatten``'s ``splitter`` argument.
    """
    def f(flat_key):
        keys = tuple(flat_key.split(delimiter))
        return keys
    return f