get "libhdr"

let rot13(x) =
    'A' <= x <= 'Z' -> (x - 'A' + 13) rem 26 + 'A',
    'a' <= x <= 'z' -> (x - 'a' + 13) rem 26 + 'a',
    x

let start() be
$(  let ch = rdch()
    if ch = endstreamch then break
    wrch(rot13(ch))
$) repeat