uca criteria

UCA: Criteria for DUCET assignments

Related: https://github.com/unicode-org/unicodetools/blob/main/docs/uca/index.md

New characters and scripts should be treated similar to existing characters and scripts.

In general, look at the pull request descriptions and commit messages for changes to unidata.txt and allkeys.txt files. For example, you could use "Blame" for a character in unidata.txt to find its history.

You can find UCA-related pull requests with this query: https://github.com/unicode-org/unicodetools/pulls?q=is%3Apr+label%3Auca

For viramas and “killers”, we have discussed using a tertiary difference for a mere shape variant, and a primary difference for a functionally different character. However, this seems complicated. See

• https://github.com/unicode-org/properties/issues/192 UCA 16: virama variants primary vs. tertiary
• https://github.com/unicode-org/unicodetools/pull/752 make 113CF TULU-TIGALARI SIGN LOOPED VIRAMA a tertiary variant of 113CE TULU-TIGALARI SIGN VIRAMA