http://www.unicode.org/reports/tr31/#Default_Identifier_Syntax defines ID_Start 
as:

> Characters having the Unicode General_Category of uppercase letters (Lu), 
> lowercase letters (Ll), titlecase letters (Lt), modifier letters (Lm), other 
> letters (Lo), letter numbers (Nl), minus Pattern_Syntax and 
> Pattern_White_Space code points, plus stability extensions. Note that “other 
> letters” includes ideographs.

What are the “stability extensions” this document refers to?

I noticed that parsing `DerivedCoreProperties.txt` for `ID_Start` leads to 
slightly different results, than parsing `UnicodeData.txt` for category names 
and then adding the categories together, minus `Pattern_Syntax` and 
`Pattern_White_Space` which you can get by parsing `PropList.txt`.

For example, U+2118 SCRIPT CAPITAL P is included in `ID_Start` as per 
`DerivedCoreProperties.txt`, but it doesn’t match any of the above categories. 
Is this an example of such a “stability extension”, or was this an oversight?

Regards,
Mathias
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