A mask of additional modifier keys to use with every keyboard character.
Emacs applies the modifiers of the character stored here to each keyboard
character it reads. For example, after evaluating the expression
(setq extra-keyboard-modifiers ?\C-x)
all input characters will have the control modifier applied to them.
Note that the character ?\C-@, equivalent to the integer zero, does
not count as a control character; rather, it counts as a character
with no modifiers; thus, setting `extra-keyboard-modifiers' to zero cancels any modification.