Don't penalize player for certain impossible guesses.
(digit (- digit-char ?0))
(correct-digit (mpuz-to-digit letter)))
(cond ((mpuz-digit-solved-p correct-digit)
- (message "%c has already been found."))
+ (message "%c has already been found." (+ correct-digit ?0)))
+ ((mpuz-digit-solved-p digit)
+ (message "%c has already been placed." digit-char))
((= digit correct-digit)
(message "%c = %c correct !" letter-char digit-char)
(mpuz-ding)