[XeTeX] Math class initialization in Unicde-aware engine

[Doug McKenna]

Joseph -

A similar ambiguity occurs later in the README.md file.  It says

- \Umathcode for all letters as TeX class 7 (var)

Does "letters" mean those code points on the TeX side with \catcode 11, or those Unicode code points labeled with 'L' in UnicodeData.txt?

If the former, then combining marks (Unicode 'M') should be entered into \Umathcode as TeX class 7; if the latter, then presumably not, though it's not clear why a math variable name can't have a combining mark.

- Doug McKenna