Comment for ISO 2022 encoding mechanism modified.
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Kenichi Handa
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1 changed files with 71 additions and 61 deletions
132
src/coding.c
132
src/coding.c
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@ -525,33 +525,37 @@ detect_coding_emacs_mule (src, src_end)
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/*** 3. ISO2022 handlers ***/
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/* The following note describes the coding system ISO2022 briefly.
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Since the intention of this note is to help in understanding of
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the programs in this file, some parts are NOT ACCURATE or OVERLY
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SIMPLIFIED. For the thorough understanding, please refer to the
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Since the intention of this note is to help understand the
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functions in this file, some parts are NOT ACCURATE or OVERLY
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SIMPLIFIED. For thorough understanding, please refer to the
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original document of ISO2022.
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ISO2022 provides many mechanisms to encode several character sets
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in 7-bit and 8-bit environment. If one chooses 7-bite environment,
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all text is encoded by codes of less than 128. This may make the
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encoded text a little bit longer, but the text gets more stability
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to pass through several gateways (some of them strip off the MSB).
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There are two kinds of character set: control character set and
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in 7-bit and 8-bit environments. For 7-bite environments, all text
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is encoded using bytes less than 128. This may make the encoded
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text a little bit longer, but the text passes more easily through
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several gateways, some of which strip off MSB (Most Signigant Bit).
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There are two kinds of character sets: control character set and
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graphic character set. The former contains control characters such
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as `newline' and `escape' to provide control functions (control
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functions are provided also by escape sequences). The latter
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contains graphic characters such as ' A' and '-'. Emacs recognizes
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functions are also provided by escape sequences). The latter
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contains graphic characters such as 'A' and '-'. Emacs recognizes
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two control character sets and many graphic character sets.
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Graphic character sets are classified into one of the following
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four classes, DIMENSION1_CHARS94, DIMENSION1_CHARS96,
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DIMENSION2_CHARS94, DIMENSION2_CHARS96 according to the number of
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bytes (DIMENSION) and the number of characters in one dimension
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(CHARS) of the set. In addition, each character set is assigned an
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identification tag (called "final character" and denoted as <F>
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here after) which is unique in each class. <F> of each character
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set is decided by ECMA(*) when it is registered in ISO. Code range
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of <F> is 0x30..0x7F (0x30..0x3F are for private use only).
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four classes, according to the number of bytes (DIMENSION) and
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number of characters in one dimension (CHARS) of the set:
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- DIMENSION1_CHARS94
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- DIMENSION1_CHARS96
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- DIMENSION2_CHARS94
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- DIMENSION2_CHARS96
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In addition, each character set is assigned an identification tag,
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unique for each set, called "final character" (denoted as <F>
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hereafter). The <F> of each character set is decided by ECMA(*)
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when it is registered in ISO. The code range of <F> is 0x30..0x7F
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(0x30..0x3F are for private use only).
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Note (*): ECMA = European Computer Manufacturers Association
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@ -561,55 +565,61 @@ detect_coding_emacs_mule (src, src_end)
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o DIMENSION2_CHARS94 -- GB2312('A'), JISX0208('B'), ...
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o DIMENSION2_CHARS96 -- none for the moment
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A code area (1byte=8bits) is divided into 4 areas, C0, GL, C1, and GR.
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A code area (1 byte=8 bits) is divided into 4 areas, C0, GL, C1, and GR.
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C0 [0x00..0x1F] -- control character plane 0
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GL [0x20..0x7F] -- graphic character plane 0
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C1 [0x80..0x9F] -- control character plane 1
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GR [0xA0..0xFF] -- graphic character plane 1
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A control character set is directly designated and invoked to C0 or
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C1 by an escape sequence. The most common case is that ISO646's
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control character set is designated/invoked to C0 and ISO6429's
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control character set is designated/invoked to C1, and usually
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these designations/invocations are omitted in a coded text. With
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7-bit environment, only C0 can be used, and a control character for
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C1 is encoded by an appropriate escape sequence to fit in the
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environment. All control characters for C1 are defined the
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corresponding escape sequences.
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C1 by an escape sequence. The most common case is that:
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- ISO646's control character set is designated/invoked to C0, and
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- ISO6429's control character set is designated/invoked to C1,
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and usually these designations/invocations are omitted in encoded
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text. In a 7-bit environment, only C0 can be used, and a control
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character for C1 is encoded by an appropriate escape sequence to
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fit into the environment. All control characters for C1 are
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defined to have corresponding escape sequences.
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A graphic character set is at first designated to one of four
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graphic registers (G0 through G3), then these graphic registers are
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invoked to GL or GR. These designations and invocations can be
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done independently. The most common case is that G0 is invoked to
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GL, G1 is invoked to GR, and ASCII is designated to G0, and usually
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these invocations and designations are omitted in a coded text.
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With 7-bit environment, only GL can be used.
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GL, G1 is invoked to GR, and ASCII is designated to G0. Usually
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these invocations and designations are omitted in encoded text.
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In a 7-bit environment, only GL can be used.
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When a graphic character set of CHARS94 is invoked to GL, code 0x20
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and 0x7F of GL area work as control characters SPACE and DEL
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respectively, and code 0xA0 and 0xFF of GR area should not be used.
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When a graphic character set of CHARS94 is invoked to GL, codes
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0x20 and 0x7F of the GL area work as control characters SPACE and
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DEL respectively, and codes 0xA0 and 0xFF of the GR area should not
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be used.
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There are two ways of invocation: locking-shift and single-shift.
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With locking-shift, the invocation lasts until the next different
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invocation, whereas with single-shift, the invocation works only
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for the following character and doesn't affect locking-shift.
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Invocations are done by the following control characters or escape
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sequences.
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invocation, whereas with single-shift, the invocation affects the
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following character only and doesn't affect the locking-shift
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state. Invocations are done by the following control characters or
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escape sequences:
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----------------------------------------------------------------------
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function control char escape sequence description
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abbrev function cntrl escape seq description
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----------------------------------------------------------------------
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SI (shift-in) 0x0F none invoke G0 to GL
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SO (shift-out) 0x0E none invoke G1 to GL
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LS2 (locking-shift-2) none ESC 'n' invoke G2 into GL
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LS3 (locking-shift-3) none ESC 'o' invoke G3 into GL
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SS2 (single-shift-2) 0x8E ESC 'N' invoke G2 into GL
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SS3 (single-shift-3) 0x8F ESC 'O' invoke G3 into GL
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SI/LS0 (shift-in) 0x0F none invoke G0 into GL
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SO/LS1 (shift-out) 0x0E none invoke G1 into GL
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LS2 (locking-shift-2) none ESC 'n' invoke G2 into GL
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LS3 (locking-shift-3) none ESC 'o' invoke G3 into GL
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LS1R (locking-shift-1 right) none ESC '~' invoke G1 into GR (*)
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LS2R (locking-shift-2 right) none ESC '}' invoke G2 into GR (*)
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LS3R (locking-shift 3 right) none ESC '|' invoke G3 into GR (*)
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SS2 (single-shift-2) 0x8E ESC 'N' invoke G2 for one char
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SS3 (single-shift-3) 0x8F ESC 'O' invoke G3 for one char
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----------------------------------------------------------------------
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The first four are for locking-shift. Control characters for these
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functions are defined by macros ISO_CODE_XXX in `coding.h'.
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(*) These are not used by any known coding system.
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Designations are done by the following escape sequences.
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Control characters for these functions are defined by macros
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ISO_CODE_XXX in `coding.h'.
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Designations are done by the following escape sequences:
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----------------------------------------------------------------------
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escape sequence description
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----------------------------------------------------------------------
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@ -632,40 +642,40 @@ detect_coding_emacs_mule (src, src_end)
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----------------------------------------------------------------------
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In this list, "DIMENSION1_CHARS94<F>" means a graphic character set
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of dimension 1, chars 94, and final character <F>, and etc.
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of dimension 1, chars 94, and final character <F>, etc...
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Note (*): Although these designations are not allowed in ISO2022,
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Emacs accepts them on decoding, and produces them on encoding
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CHARS96 character set in a coding system which is characterized as
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CHARS96 character sets in a coding system which is characterized as
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7-bit environment, non-locking-shift, and non-single-shift.
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Note (**): If <F> is '@', 'A', or 'B', the intermediate character
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'(' can be omitted. We call this as "short-form" here after.
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'(' can be omitted. We refer to this as "short-form" hereafter.
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Now you may notice that there are a lot of ways for encoding the
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same multilingual text in ISO2022. Actually, there exists many
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coding systems such as Compound Text (used in X's inter client
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communication, ISO-2022-JP (used in Japanese Internet), ISO-2022-KR
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(used in Korean Internet), EUC (Extended UNIX Code, used in Asian
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same multilingual text in ISO2022. Actually, there exist many
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coding systems such as Compound Text (used in X11's inter client
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communication, ISO-2022-JP (used in Japanese internet), ISO-2022-KR
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(used in Korean internet), EUC (Extended UNIX Code, used in Asian
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localized platforms), and all of these are variants of ISO2022.
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In addition to the above, Emacs handles two more kinds of escape
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sequences: ISO6429's direction specification and Emacs' private
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sequence for specifying character composition.
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ISO6429's direction specification takes the following format:
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ISO6429's direction specification takes the following form:
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o CSI ']' -- end of the current direction
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o CSI '0' ']' -- end of the current direction
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o CSI '1' ']' -- start of left-to-right text
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o CSI '2' ']' -- start of right-to-left text
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The control character CSI (0x9B: control sequence introducer) is
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abbreviated to the escape sequence ESC '[' in 7-bit environment.
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Character composition specification takes the following format:
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abbreviated to the escape sequence ESC '[' in a 7-bit environment.
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Character composition specification takes the following form:
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o ESC '0' -- start character composition
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o ESC '1' -- end character composition
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Since these are not standard escape sequences of any ISO, the use
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of them for these meaning is restricted to Emacs only. */
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Since these are not standard escape sequences of any ISO standard,
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the use of them for these meaning is restricted to Emacs only. */
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enum iso_code_class_type iso_code_class[256];
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