rot13 = proc (c: char) returns (char)
    base: int
    letter: bool := false
    if c>='A' & c<='Z' then
        base := char$c2i('A')
        letter := true
    elseif c>='a' & c<='z' then
        base := char$c2i('a')
        letter := true
    end
    if ~letter then return(c) end
    return(char$i2c((char$c2i(c)-base+13)//26+base))
end rot13

start_up = proc ()
    po: stream := stream$primary_output()
    pi: stream := stream$primary_input()
    while true do
        stream$putc(po,rot13(stream$getc(pi)))
        except when end_of_file: break end
    end
end start_up