def _parse_letters(letters):
    """
    Turns a string of residue letters (i.e. "STY") into a list.

    Includes special provisions for N- and C-term modifications.

    Parameters
    ----------
    letters : str

    Returns
    -------
    tuple of str
    """
    if letters in ['N-term', 'C-term']:
        return letters,
    return tuple(letters)
