It just makes the natural numbers

seq.py

class NegativeStep(Exception):
    pass

def countdown(n,step=0):
    if n==0:
        return [0]
    if n<0:
        raise NegativeStep()
        
    for d in range(-1,2):
        try:
            s = step+d
            if s < 0:
                return [n]+countdown(n+s, s)
        except NegativeStep:
            continue

    raise NegativeStep()

def runs(seq):
    last = None
    t = 0
    for x in seq:
        if x != last:
            if t > 0:
                yield t
            t = 0
            last = x
        else:
            t += 1

list(runs([len(countdown(n)) for n in range(1,50)]))