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I was trying to follow a solution from Quinten to a problem about summing probability amplitudes and was tripped up by a part that goes from the second to the third equation here. After a lot of pondering, I gave up, then asked her about it at the meeting.
It turned out I misread a minus sign next to a fraction as being part of the fraction line, so that the fraction was multiplying the rest of it instead the fraction begin subtracted from the rest of it.