Emacs prefix key error: why is M-z considered a sequence?

Question

I want to set viper-toggle-key to M-z (C-z is undo and only undo!). Looks easy: I just need to add (setq viper-toggle-key [(meta ?z)]) in .viper. Except I get this message:

Warning (initialization): An error occurred while loading `d:/Alexey/Home/.emacs.d/init.el':

error: Key sequence M-z starts with non-prefix key

I don't understand why: M-z is a single key, not a sequence!

Answer 1

M is the meta key, which is usually not a modifier (like control) - typically it's something like ESC, which you type prior to the other character, e.g. ESC then z. So meta key combinations are, in general, sequences, rather than single keypresses.