/r13 {
4 dict begin
    /rotc {
        {
        {{{64 gt} {91 lt}} all?} {65 - 13 + 26 mod 65 +} is?
        {{{95 gt} {123 lt}} all?} {97 - 13 + 26 mod 97 +} is?
        } cond
    }.
    {rotc} map cvstr
end}.