(2) Amount of Carrier-Caused Refunds
b) When a portion of the trip has been made and the passenger elects to continue to destination by travel not arranged by carrier, the amount of refund of the unused portion will be prorated based on mileage.
This seems to indicate that if I only use MAD-YYZ and elect to continue on my own from YYZ-BOS, I would receive a partial refund for the YYZ-BOS leg (it does not mention repricing - am I missing something??)