Python itertools - combination logic

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Python

I'm trying to understand the combination function in itertools, but I'm stuck with one of the loops. Beyond the else statement, I can't follow what's going on. Do the "indices[i] += 1", and for loop fall into the return function?

Also, why does the code need to stipulate the j + 1 = j-1+1?

def combinations(iterable, r):
    # combinations('ABCD', 2) --> AB AC AD BC BD CD
    # combinations(range(4), 3) --> 012 013 023 123
    pool = tuple(iterable)
    n = len(pool)
    if r > n:
	    print("oops")
    indices = list(range(r))
    yield tuple(pool[i] for i in indices)
    while True:
        for i in reversed(range(r)):
            if indices[i] != i + n - r:
                break
        else:
            return
        indices[i] += 1
        for j in range(i+1, r):
            indices[j] = indices[j-1] + 1
        yield tuple(pool[i] for i in indices)

What I have tried:

I understand return and else statements overall, and I've worked through the code up until this point. I'm just looking for some help here.