`variable-not-in` uses wrong prefix when prefix ends in a number

[florence]

(variable-not-in 'S3 'S3)

Gives back S1, rather than S31.

While this still does give back unique identifiers it can make the output harder to read in some cases.

[florence]

An interesting corner case here:

> (variable-not-in '|1| '|1|)
'|12|

Which is the behavior I would expect (if a tad of a silly example).