# your code goes here
lst = (1, (2, 3), (4, 5), ((6, 7)), (8))
def gen(lst):
	for l in lst:
		(yield from gen(l)) if isinstance(l, tuple) else (yield l)
 
def serial(lst):
	prev = 0
	for i in gen(lst):
		yield i, prev
		prev = i
 
print(list(serial(lst)))
 
def parse(l, *rest, prev=0):
	ret = parse(*l, prev=prev) if isinstance(l, tuple) else [(l, prev)]
	ret += parse(*rest, prev=ret[-1][0]) if rest else []
	return ret
 
#return ([i for j in l for i, last in recurse(j)], last) if isinstance(l, tuple) else [l], last
print(parse(lst))

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