comment
  Return the rot13 transformation of s, preserving case and
  passing non-alphabetic characters without change
end

function rot13(s = string) = string
  var i, k = integer
  var ch = char
  var normal, rotated = string
  normal  = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
  rotated = "NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm"
  for i = 1 to len(s)
    ch = mid(s,i,1)
    k = instr(1,normal,ch)
    if k <> 0 then ch = mid(rotated,k,1)
    mid(s,i,1) = ch
  next i
end = s

rem - exercise the function

var plain, encoded = string

plain = "The quick brown fox jumps over the lazy dog."
encoded = rot13(plain)
print "Plain text: "; plain
print "Encoded   : "; encoded
print "Decoded   : "; rot13(encoded)

end