Thank you for the swift reply! You helped me much.

Incidentally, in my search I found "changevar" but the title said "deprecated", so I skipped it entirely.

I believe the justification for the last step is, since we are summing from -inf to inf, we may do it in what order we like, so whether it's q-s or q is irrelevant. At any rate, I've seen it in a few textbooks.

Thank you for the swift reply! You helped me much.

Incidentally, in my search I found "changevar" but the title said "deprecated", so I skipped it entirely.

I believe the justification for the last step is, since we are summing from -inf to inf, we may do it in what order we like, so whether it's q-s or q is irrelevant. At any rate, I've seen it in a few textbooks.