def fib(n, b, l):
    xs = [[1, 0]]
    for _ in range(n + 1):
        new_xs = []
        ad = 0
        for x, y in xs:
            if y < l:
                new_xs.append([x, y + 1])
        for x, y in new_xs:
            if y > 1:
                ad += x 
        if ad > 0:
            new_xs.append([ad * b, 0])
        xs = new_xs
        print("adults :", ad, xs)  
    print(xs)
    return sum(x for x, y in xs if y > 0)
r = fib(8, 3, 4)
print(r)
[[12, 4], [18, 3], [54, 2], [99, 1], [252, 0]]