def countdown(k):
    if k > 0:
        yield k
        yield from countdown(k - 1)
    else:
        yield 'Blast off'

def prefixes(s):
    if s:
        yield from prefixes(s[:-1])
        yield s

def substrings(s):
    if s:
        yield from prefixes(s)
        yield from substrings(s[1:])

In [2]: (list(countdown(5)),list(prefixes('both')),list(substrings('tops')))
Out[2]:
([5, 4, 3, 2, 1, 'Blast off'],
 ['b', 'bo', 'bot', 'both'],
 ['t', 'to', 'top', 'tops', 'o', 'op', 'ops', 'p', 'ps', 's'])