def memo(f):
    """Return a function that behaves like f but caches return values.

    Note: This version accepts a function of any number of arguments. The
    simpler version in the slides accepts functions of only one argument.
    """
    cache = {}
    def memoized(*args):
        if args not in cache:
            cache[args] = f(*args)
        return cache[args]
    return memoized

def fib(n):
    """Return the nth Fibonacci number

    >>> fib(6)
    5
    """
    if n == 1:
        return 0
    if n == 2:
        return 1
    return fib(n-2) + fib(n-1)

def fib_iter(n):
    """Return the nth Fibonacci number

    >>> fib_iter(6)
    5
    """
    prev, curr = 1, 0
    for _ in range(n-1):
         prev, curr = curr, prev + curr
    return curr

def count_change(a, kinds=(50, 25, 10, 5, 1)):
    """Return the number of ways to change amount a using coin kinds.

    >>> count_change(30)
    18
    """
    if a == 0:
        return 1
    if a < 0 or len(kinds) == 0:
        return 0
    d = kinds[0]
    return count_change(a, kinds[1:]) + count_change(a - d, kinds)