rem  Return the ROT13 transformation of s$, preserving case \
     and passing non-alphabetic characters without change

def fn.rot13$(s$)
  normal$ ="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
  rotated$="NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm"
  outstr$ = ""
  for i% = 1 to len(s$)
    c$ = mid$(s$,i%,1)
    k% = match(c$,normal$,1)
    if k% <> 0 then c$ = mid$(rotated$,k%,1)
    outstr$ = outstr$ + c$
  next i%
fn.rot13$ = outstr$
return
fend

plain$ = "The quick brown fox jumps over the lazy dog."
encoded$ = fn.rot13$(plain$)

print "Plain Text: "; plain$
print "Encoded   : "; encoded$
print "Restored  : "; fn.rot13$(encoded$)

end