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The traveling salesperson problem with the additional
condition that certain pairs of stops must be visited in a given order.
Specifically, for i even, stop i must be visited before stop i+1.
The interpretation is that someone is to be picked up at i and later
dropped off at i+1. The vehicle has sufficient capacity so that it
can carry as many people as necessary.
Stop 1 is the home base where no one is picked up and no one is dropped off.