Don't know if I'm fully following this or understand exactly what a Ruby Fiber is ...

But it did inspire me to come up with this little bit of Python black-magic :

def makeSection(f) :
    def gen(source) :
        for x in source :
            yield f(x)
    return gen


def makeFilter(test) :
    def gen(source) :
        for x in source :
            if test(x) : yield x
    return gen

    
def pipeline(generators) :
    def p(source) :
        old = source
        for g in generators :
            new = g(old)
            old = new
        return old
    return p
    
t2 = makeSection(lambda x : x * 2)
t3 = makeSection(lambda x : x * 3 )
f1 = makeFilter(lambda x : (x>20))
f2 = makeFilter(lambda x : (x<50))

pipe = pipeline( [ t2 , t3 , f1 , f2 ] )

for x in pipe(range(10)) : print x

See if you can work it out, I'll probably go over it tomorrow ...