# iterator example; uses Fibonacci numbers, so common in computer
# science examples: f_n = f_{n-1} + f_{n-2}, with f_0 = f_1 = 1

class fibnum:
    def __init__(self):
        self.fn2 = 1 # "f_{n-2}"
        self.fn1 = 1 # "f_{n-1}"

    def next(self): # next() is the heart of any iterator
        # note the use of the following tuple to not only save lines of
        # code but also to insure that only the old values of self.fn1 and
        # self.fn2 are used in assigning the new values
        (self.fn1,self.fn2,oldfn2) = (self.fn1+self.fn2,self.fn1,self.fn2)
        return oldfn2

    def __iter__(self):
         return self


class fibnum20:
    def __init__(self):
        self.fn2 = 1 # "f_{n-2}"
        self.fn1 = 1 # "f_{n-1}"

    def next(self):
        (self.fn1,self.fn2,oldfn2) = (self.fn1+self.fn2,self.fn1,self.fn2)
        if oldfn2 > 20: raise StopIteration
        return oldfn2

    def __iter__(self):
        return self