Imagine that you are travelling north on an interstate highway, and the median is blocked by a concrete wall. You can turn around, change direction, only when you reach an interchange. The interchange represents an opportunity to change direction. You may or may not use the opportunity to change direction. If you do not change direction upon reaching the interchange you must continue north until you reach the next interchange. Once again, the interchange merely represents an opportunity to turn, and you may pass many interchanges without changing direction.