// An action transitions stochastically to a resulting score.
type action func(current score) (result score, turnIsOver bool)
-// roll returns the (result, turnIsOver) outcome of simulating a die roll.
+// roll returns the (result, turnIsOver) outcome of simulating a die roll.
// If the roll value is 1, then thisTurn score is abandoned, and the players'
// roles swap. Otherwise, the roll value is added to thisTurn.
func roll(s score) (score, bool) {