What if I don't have an <Escape> key?
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Type `C-[' instead. This should send ASCII code 27 just like an
Escape key would. `C-3' may also work on some terminal (but not under
X). For many terminals (notably DEC terminals) <F11> generates <ESC>.
If not, the following form can be used to bind it:
;;; F11 is the documented ESC replacement on DEC terminals.
(define-key function-key-map [f11] [?\e])