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Proof-of-payment or POP is an honor-based fare collection approach used on many public transportation systems. Instead of checking each passenger as they enter a fare control zone, proof-of-payment requires that each passenger carry a ticket or pass proving that they have paid the fare. Ticket controller or conductors make periodic checks to deter fare evasion. On many systems, a passenger can purchase a single use ticket or multi-use pass at any time in advance, but must insert the ticket or pass into a validation machine before use. Validation machines in stations or on board vehicles time stamp the ticket. The ticket is then valid for some period of time after the stamped time.