http://userguide.icu-project.org/collation/api

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ICU comes with a large set of already predefined collators that are suited for specific locales. Most of the ICU locales have a predefined collator. In worst case, the default set of rules, which is equivalent to the UCA ordering, is used.

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ICUはライブラリとしてロケールごとのcollationを規定しているがデフォルトではUCA順序になる（RootLocale？

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http://www.unicode.org/reports/tr10/

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Unicode® Technical Standard #10 UNICODE COLLATION ALGORITHM