List of approved standard encodings (i.e. charsets) of X's Compound Text.
Coding-system `compound-text-with-extensions' encodes a character
belonging to any of those charsets using the normal ISO2022
designation sequence unless the current language environment or
the variable `ctext-non-standard-encodings' decide to use an extended
segment of CTEXT for that character. See also the documentation
of `ctext-non-standard-encodings-alist'.