Constraint is number of inputs that yield the same output
Luis describes peekaboom and verbosity as “asymmetric verification games”: Input is given to player 1, whose output is sent to player 2, who has to guess the input given only player 1's output. Once he guesses, this verifies the connection between the output and the input. This is in contrast to the ESP Game, which is a symmetric verification game. But symmetric games work only when there is a constrained list of possible outputs.