ch@(String Character traits) rot13
[| value |
  upper ::= ch isUppercase.
  value := ch toLowercase as: Integer.
  (value >= 97) /\ [value < 110]
    ifTrue: [value += 13]
    ifFalse: [(value > 109) /\ [value <= 122]
                ifTrue: [value -= 13]].
  upper
    ifTrue: [(value as: String Character) toUppercase]
    ifFalse: [value as: String Character]
].

s@(String traits) rot13
[
  result ::= s newSameSize.
  s doWithIndex: [| :each :index | result at: index put: each rot13].
  result
].

slate[37]> 'abc123' rot13.
'nop123'