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

print list(enumerate("Hello"))


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

#print list(tee("Hello!!", "World"))
print tee("", "!!").next()

def empty():
    if False:
        yield None


from py.magic import greenlet
# ...