Tests that exactly N items in an iterable are "truthy" (neither None,
    False, nor 0).

def exactly_n(l, n=1):
    '''
    Tests that exactly N items in an iterable are "truthy" (neither None,
    False, nor 0).
    '''
    i = iter(l)
    return all(any(i) for j in range(n)) and not any(i)