Reference Label Generation Rules for the Lao Script

Overview

This document specifies a reference set of Label Generation Rules (LGR) for the Lao script for the second level. The starting point for the development of this LGR can be found in the related Root Zone LGR [RZ-LGR-3-Laoo]. For details and additional background on the script, see "Proposal for a Lao Script Root Zone LGR [Proposal-Lao]". The format of this file follows [RFC 7940].

This is a DRAFT document released for public comments and not final. Please see the announcement on the ICANN website for public comments on the Second Level Reference LGRs for details on how to submit comments.

Repertoire

The repertoire contains 51 code points for letters; in addition, the sequence 0EB2 0EB0 has been defined to facilitate implementation of WLE rule follows-vafter-context as a context rule. The repertoire only includes code points used by languages that are actively written in the Lao script. The repertoire is a subset of [Unicode 6.3]. For details, see Section 5 “Repertoire” in [Proposal-Lao]. (The proposal cited has been adopted for the Lao script portion of the Root Zone LGR.)

For the second level, the repertoire has been augmented with the ASCII digits, U+0030 (0) to U+0039 (9), Lao digits, U+0ED0 (໐) to U+0ED9 (໙), and U+002D (-) HYPHEN-MINUS for a total of 73 repertoire elements.

Each code point or range is tagged with the script or scripts that the code point is used with, one or more categories, and one or more references documenting sufficient justification for inclusion in the repertoire, see "References" below.

Variants

This LGR defines no variants for letters. See Section 6, "Variants" in [Proposal-Lao].

Digit Variants: All Lao digits are treated as semantic variants of the corresponding common (ASCII) digits. By transitivity, they are also semantic variants of any native digits in scripts that also include the common digits. Such transitive relations are deemed to exist implicitly but are not listed explicitly in each reference LGR. (Omitting the listing of these other cross script digit variants does not affect index variant calculation, as the ASCII digit variant being smallest would always be the index variant.) There is a strong resemblance between Thai and Khmer digits, and certain Lao digits. In addition, Lao digit ZERO is a cross-script homoglyph or near homoglyph of digit ZERO in many other scripts; all of these are already implicit semantic variants by transitivity and therefore not listed here. To keep digit variant sets manageable in zones where multiple scripts are present, no attempt has been made at identifying cross-script variants among digits of different numeric value or between a digit in one script and a letter in another, such as between digit zero and Latin letter 'o'. Other mechanisms may be required to prevent homograph labels.

Character Classes

Consonants: In regular syllables, consonants occur in limited combinations. However, arbitrary combinations are used for acronyms. The LGR therefore considers the restriction on syllabic combinations a matter of spelling and does not enforce them. Consonants may be followed by a semi-consonant mark. Some consonants have been given the tag "Cf", which indicates final consonants. See Section 5, "Consonants" in [Proposal-Lao].

Vowels: Vowels are divided into vowel-above, vowel-before, vowel-below and vowel-after so as to enforce some of the syllable structure using context rules. However, many details have been considered spelling issues and, for simplification, are not modeled in this LGR. See Section 5 in [Proposal-Lao].

Semi-consonant: The character U+0EBC (ຼ) LAO SEMIVOWEL SIGN LO follows consonants (see Section 5 in [Proposal-Lao]).

Tone-mark: Any of four tone marks can follow a consonant or vowel-above or vowel-below (see Section 5 in [Proposal-Lao]).

Signs: The character U+0ECC (໌) LAO CANCELLATION MARK follows a final consonant (Cf). The character U+0EC6 (ໆ) LAO KO LA is a repetition mark that can only occur up to 3 times at the end of the label (See Section 5 in [Proposal-Lao]).

Lao Digits: U+0E50 (๐) to U+0E59 (๙) are a set of Lao-specific digits. They are used in alternation with the European (common) digits.

Common Digits: U+0030 (0) to U+0039 (9) are the set of digits from the ASCII range.

Whole Label Evaluation (WLE) and Context Rules

Common Rules

• Hyphen Restrictions — restrictions on the allowable placement of hyphens (no leading/ending hyphen and no hyphen in positions 3 and 4). These restrictions are described in section 4.2.3.1 of RFC 5891 [150]. They are implemented here as context rule on U+002D (-) HYPHEN-MINUS.
• Leading Combining Marks — restrictions on the allowable placement of combining marks (no leading combining mark). This rule is described in section 4.2.3.2 of RFC 5891 [150].
• One non-ASCII code point — An IDN label must contain one non-ASCII code point, as described in Section 4.2.4 of RFC 5891 [150].
• Digit-mixing — no mixing between different digit sets (European and Lao digits) is allowed; implemented here as a WLE rule with associated action.

Default Actions

Actions include the default actions for LGRs as well as that needed to invalidate labels with misplaced combining marks. They are marked with ⍟. For a description see [RFC 7940].

Lao-specific Rules

Rules provided in the LGR as described in Section 7 of [Proposal-Lao] reasonably restrict labels so that they conform to Lao syllable structure. These constraints are presented exclusively as context rules.

The rules are:

• follows-consonant — A context rule for semi-consonant. See Section 7 in [Proposal-Lao]. (WLE Rule 1)
• precedes-consonant — A context rule for vowel-before. See Section 7 in [Proposal-Lao]. (WLE Rule 2)
• follows-main-consonant — A context rule for vowel-below, and vowel-above. See Section 7 in [Proposal-Lao]. (WLE Rule 3)
• follows-C-tonemark-vabove — A context rule for vowel-after. See Section 7 in [Proposal-Lao]. (WLE Rule 4)
• follows-vbefore-consonant-cluster — A context rule for a vowel-after sequence. It incorporates consonant-cluster. See Section 7 in [Proposal-Lao]. (WLE Rule 5)
• follows-C-vabove-vbelow — A context rule for tone mark. See Section 7 in [Proposal-Lao]. (WLE Rule 6)
• follows-Cf — A context rule for U+0ECC (໌ ) LAO CANCELLATION MARK. See Section 7 in [Proposal-Lao]. (WLE Rule 7)
• repetition-mark-limit — A rule that limits the occurrence of U+0EC6 ( ໆ ) LAO KO LA at the label end. See Section 7 in [Proposal-Lao]. (WLE Rule 8)

No context rules apply to “consonant” code points. For discussion, see Section 5.1, “Consonants” in [Proposal-Lao].

Methodology and Contributors

This reference LGR for Lao for the 2nd Level has been developed by Michel Suignard and Asmus Freytag, based on the Root Zone LGR for Lao and information contained or referenced therein, see [RZ-LGR-3-Laoo]. Suitable extensions for the second level have been applied according to the [Guidelines]. The original proposal for a Root Zone LGR for the Lao script, that this reference LGR is based on, was developed by the Lao Generation Panel. For more information on methodology and contributors to the underlying Root Zone LGR, see Sections 4 and 8 in [Proposal-Lao], as well as [RZ-LGR-Overview].

References

The following general references are cited in this document:

[Guidelines]

ICANN, "Guidelines for Developing Reference LGRs for the Second Level, (Los Angeles, California: ICANN, 27 May 2020),
https://www.icann.org/en/system/files/files/lgr-guidelines-second-level-06jan16-en.pdf

[Proposal-Lao]

Lao Generation Panel, "Proposal for Lao Script Root Zone LGR",
https://www.icann.org/en/system/files/files/proposal-lao-lgr-31jan17-en.pdf

[RFC 7940]

Davies, K. and A. Freytag, "Representing Label Generation Rulesets Using XML", RFC 7940, August 2016,
http://www.rfc-editor.org/info/rfc7940

[RZ-LGR-Overview]

Integration Panel, "Root Zone Label Generation Rules - LGR-3: Overview and Summary", 01 March 2020 (PDF),
https://www.icann.org/sites/default/files/lgr/lgr-3-overview-10jul19-en.pdf

[RZ-LGR-3-Laoo]

ICANN, Root Zone Label Generation Rules for the Lao Script (und-Laoo) , 10 July 2019 (XML)
https://www.icann.org/sites/default/files/lgr/lgr-3-lao-script-10jul19-en.xml

[Unicode 6.3]

The Unicode Consortium. The Unicode Standard, Version 6.3.0, (Mountain View, CA: The Unicode Consortium, 2013. ISBN 978-1-936213-08-5)
http://www.unicode.org/versions/Unicode6.3.0/

For references consulted particularly in designing the repertoire for the Lao script for the second level. please see details in the Table of References below. Reference [0] refers to Unicode Standard version in which corresponding code points were initially encoded. References [201], [202], [203], [204], 205], [206], & [207] correspond to sources justifying the inclusion of or classification for the corresponding code points. Entries in the table may have multiple source reference values.

4.1 Character Classes

The following table lists all named and implicit classes with their definition and a list of their members intersected with the current repertoire (for larger classes, this list is elided).

4.2 Whole label evaluation and context rules

The following table lists all named rules defined in the LGR and indicates whether they are used as trigger in an action or as context (when or not-when) for a code point or variant.

4.3 Actions

The following table lists the actions that are used to assign dispositions to labels and variant labels based on the specified conditions. The order of actions defines their precedence: the first action triggered by a label is the one defining its disposition.