Two friends live in two different ends of a very long bridge. At any point in time, only one of them can cross the bridge to prevent overloading. They cannot see whether the other is currently crossing the bridge (it's a very long bridge!). Therefore, they agreed upon a rule: whoever wants to cross the bridge would follow the steps below.

  1. Start a smoke signal.
  2. If smoke is present on the other side, wait.
  3. Cross the bridge, return and stop the signal.

If they follow the rule, which scenario could happen?


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