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Switching to path planning, this map shows the routes involved in the 2,000 quickest paths (flight combinations) from San Francisco to Boston on a certain day. Specific flights are not shown: some arcs represent multiple flights between the same airports at different times of day. Notice that all of these paths look reasonable: they don’t leave the United States and southern Canada, all are length 3 or less, and all arrive the same day. An impecunious traveler might be willing to consider any of them. But 2,000 paths doesn’t begin to exhaust the possibilities.