def descent_set(perm):
    """Returns descent set of the permutation

    >>> Permutation(42561873).descent_set()
    [1, 4, 6, 7]
    """
    p = list(perm)
    n = perm.__len__()
    descents = []
    for i in range(1, n):
        if p[i - 1] > p[i]:
            descents.append(i)
    return descents
