Why does Emacs say "Key sequence XXX uses invalid prefix characters"?
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Usually, one of two things has happened. In one case, the control
character in the key sequence has been misspecified (e.g. `C-f' used
instead of `\C-f' within a Lisp expression). In the other case, a
"prefix key" in the keystroke sequence you were trying to bind was
already bound as a "complete key". Historically, the `ESC [' prefix
was usually the problem, in which case you should evaluate either of
these forms before attempting to bind the key sequence:
(global-unset-key [?\e ?[]) ;; or
(global-unset-key "\e[")
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