def interval_diff(progression1, progression2, interval):
    """Return the number of half steps progression2 needs to be diminished or
    augmented until the interval between progression1 and progression2 is
    interval."""
    i = numeral_intervals[numerals.index(progression1)]
    j = numeral_intervals[numerals.index(progression2)]
    acc = 0
    if j < i:
        j += 12
    while j - i > interval:
        acc -= 1
        j -= 1
    while j - i < interval:
        acc += 1
        j += 1
    return acc
Return the number of half steps progression2 needs to be diminished or
    augmented until the interval between progression1 and progression2 is
    interval.