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n dwarves have been made prisoners by an ogre. They will be kept in separate cells with no means of communication. The ogre will call each dwarf into a room with an electric light. They will be allowed to switch it on or off − or to do nothing. The dwarves will be made free if and only if at some point, one of them rightfully announces: "All of us have walked into this room". They are only allowed one attempt, of course. Until then, each dwarf will be brought to the room indefinitely, i.e. an infinite number of times if no dwarf can announce the result. We don't know the order the ogre has chosen. The dwarves are only allowed to communicate once, before being secluded in their cells for a potential eternity.