def _get_unused_letter(used_letters):
    doubles = [first + second for second in string.ascii_lowercase
               for first in string.ascii_lowercase]
    all_letters = set(list(string.ascii_lowercase) + doubles)
    letters = list(used_letters - all_letters)
    # NOTE(vish): prepend ` so all shorter sequences sort first
    letters.sort(key=lambda x: x.rjust(2, '`'))
    return letters[0]
dataset/ETHPy150Open BU-NU-CLOUD-SP16/Trusted-Platform-Module-nova/nova/compute/utils.py/_get_unused_letter/swapped operands of `-`