PROBLEM A: The Keep-Right-Except-To-Pass Rule

靠右除了超车规则
In countries where driving automobiles on the right is the rule (that
is, USA, China and most other countries except for Great Britain,
Australia, and some former British colonies), multi-lane freeways
often employ a rule that requires drivers to drive in the right-most
lane unless they are passing another vehicle, in which case they move
one lane to the left, pass, and return to their former travel lane.
在一些开车靠右行驶的国家（美国，中国和大多数的其他国家，除了英国，澳
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