vrmiguel

    % cat sum.py 
    def sum(depth, x):
        if depth == 0:
            return x
        else:
            fst = sum(depth-1, x*2+0) # adds the fst half
            snd = sum(depth-1, x*2+1) # adds the snd half
        return fst + snd

    if __name__ == '__main__':
        print(sum(30, 0))

    % time pypy sum.py
    576460751766552576
    pypy sum.py  4.26s user 0.06s system 96% cpu 4.464 total