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flatlines
Every buy is a heartbeat. Ten minutes without one and it flatlines. The next buy shocks it back.
Any buy of $CPR, any size, lands on the monitor as a beat and resets the clock to 10:00.
If nobody buys for ten minutes, the line goes flat and the time of death is stamped on the monitor for everyone to see.
Whoever buys first after a flatline revives it. Their name goes on the revivals board, with how long it was gone.