increment left col ("carry") <- '0' -> '1' -> .. '9' (digits)
                                 ^                |
                                 |                |
                                 +----------------+

increment left col ("carry") <- 'a' -> 'b' -> .. 'z' (lower case)
                                 ^                |
                                 |                |
                                 +----------------+

increment left col ("carry") <- 'A' -> 'B' -> .. 'Z' (upper case)
                                 ^                |
                                 |                |
                                 +----------------+