Alist of non-standard encoding names vs the corresponding usages in CTEXT.
It controls how extended segments of a compound text are handled
by the coding system `compound-text-with-extensions'.
Each element has the form (ENCODING-NAME CODING-SYSTEM N-OCTET CHARSET).
ENCODING-NAME is an encoding name of an "extended segment".
CODING-SYSTEM is the coding-system to encode (or decode) the
characters into (or from) the extended segment.
N-OCTET is the number of octets (bytes) that encodes a character
in the segment. It can be 0 (meaning the number of octets per
character is variable), 1, 2, 3, or 4.
CHARSET is a character set containing characters that are encoded
in the segment. It can be a list of character sets.
On decoding CTEXT, all encoding names listed here are recognized.
On encoding CTEXT, encoding names in the variable
`ctext-non-standard-encodings' (which see) and in the information
listed for the current language environment under the key `ctext-non-standard-encodings' are used.