Fig. 1

Four classes of trips between two points. Class 1 is the most direct trip, leaving x in the direction of y and not going past y before finishing ($xBy$). Class 2 leaves x in the other direction, but finishes when it meets y ($xABy$). Class 3 moves from x towards y, but goes past y and changes direction immediately after passing it before finishing ($xBCy$). Class 4 trips move from x first away from point y and pass y before reflecting on the next node and finishing ($xABCy$)