"Run with GNU Smalltalk: gst rot13.st -aQ [<file>]"

"This method should work for any Smalltalk in any case"
!Character methodsFor: 'ruling the universe'!
rot13
    ((self >= $a) & (self < $n)) | ((self >= $A) & (self < $N))
        ifTrue: [ ^ (self asInteger + 13) asCharacter ].
    ((self > $m) & (self <= $z)) | ((self > $M) & (self <= $Z))
        ifTrue: [ ^ (self asInteger - 13) asCharacter ].
    ^ self
! !

(Smalltalk arguments size > 0)
    ifTrue: [ (FileStream open: (Smalltalk arguments at: 1) mode: #read)
                  do: [:x | stdout << (x rot13) ]]
    ifFalse: [ stdin do: [:x | stdout << (x rot13) ]]!