callable generator

gun.py

class Gun:
    '''
    >>> g = Gun()
    >>> f = g()
    >>> [next(f), next(g)]
    [2, 1]
    >>> [next(f), next(g)]
    [3, 2]
    >>> [next(f), next(g)]
    [4, 3]
    >>> [next(f), next(g)]
    [5, 4]
    '''
    def __init__(self, start=0):
        self.start = start

    def __call__(self):
        return Gun(self.start+1)

    def __next__(self):
        self.start += 1
        return self.start