def _make_tied_note_id(prev_id):
    """Create a derived note ID for newly created notes, by appending
    letters to the ID. If the original ID has the form X-Y (e.g.
    n1-1), then the letter will be appended to the X part.

    Parameters
    ----------
    prev_id : str
        Original note ID

    Returns
    -------
    str
        Derived note ID

    Examples
    --------
    >>> _make_tied_note_id('n0')
    'n0a'
    >>> _make_tied_note_id('n0a')
    'n0b'
    >>> _make_tied_note_id('n0-1')
    'n0a-1'

    """
    prev_id_parts = prev_id.split('-', 1)
    prev_id_p1 = prev_id_parts[0]
    if prev_id_p1:
        if ord(prev_id_p1[-1]) < ord('a') - 1:
            return '-'.join(['{}a'.format(prev_id_p1)] + prev_id_parts[1:])
        else:
            return '-'.join(['{}{}'.format(prev_id_p1[:-1], chr(ord(prev_id
                [-1]) + 1))] + prev_id_parts[1:])
    else:
        return None
