def g(yield_fkn,x):

    yield_fkn(x+1)

    yield_fkn(x+2)

and 

def f(l):

   for x in l:

        g(yield,x)

and this extension works like this in python on guile

list(f([1,2])

--> [1,2,2,3]

Nifty right. This is thanks to the superb infrastructure in scheme (selimited continuations). Not only this, in python-on-guile the generators and iterators are serialisable and hence you can deep copy them. (on the condition that you turn off unboxing in the compiler).