Mapping to F13-F24?

[097115]

Let's say I have the following mapping:

define_keymap(re.compile("xterm"), {
    K("RAlt-SPACE") : K("F12"),
})

And when I launch cat in xterm, and press RAlt-SPACE, it indeed reports ^[[24~.

However, if I try F13-F24 instead of F1-F12, then cat (or xxd) shows nothing. Am I missing something?