(play-with-turns (make-game-state 5 8) 'humman)
ERROR: player wasn't human or computer: humman ← green output from program
error causes computation to halt with
its arguments printed to the terminal as
a side effect.
The first argument to error is an example of a character string.
error is the text's first example of simple I/O (input/output)
error is provided by most implementations of Scheme.