def fibonacci():
    a = 1
    b = 1
    while True:
        yield a
        (a, b) = (b, a + b)

def count(start):
    while True:
        yield start
        start += 1

def primes():
    known_primes = [2]
    yield 2
    for x in count(3):
        for div in known_primes:
            if x % div == 0:
                break
        else:
            yield x
            known_primes.append(x)

def filter_odd(gen):
    for x in gen:
        if x % 2 == 0:
            yield x

def filter(gen, condition):
    for x in gen:
        if condition(x):
            yield x

def enumerate(gen):
    i = 0
    for x in gen:
        yield (i, x)
        i += 1

def tee(gen1, gen2):
    for x1 in gen1:
        x2 = gen2.next()
        yield (x1, x2)