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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _family_notes_path(family, data_dir):
    '''Form a path to the notes for a family'''

    data_dir = fix_data_dir(data_dir)

    family = family.lower()
    if not family in get_families(data_dir):
        raise RuntimeError("Family '{}' does not exist".format(family))

    file_name = 'NOTES.' + family.lower()
    file_path = os.path.join(data_dir, file_name)
    return file_path