How does one activate a stylistic set in plain XeTeX?

I have tried:

\font\test="XITS Math:+ss01" % for calligraphic
\textfont2=\test
$\fam2 A$
\bye


but with no luck.

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Of course A gives an A, as any font has an uppercase A in the corresponding slot:

\font\testa="XITS Math:script=math;language=DFLT;"
\font\testb="XITS Math:script=math;language=DFLT;+ss01;"

\textfont2=\testa
$\XeTeXmathchar "0 "2 "1D49C$

\textfont2=\testb
$\XeTeXmathchar "0 "2 "1D49C$
\bye


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Umm, yeah, the question wasn't perhaps perfectly clear: I was trying to achieve a calligraphic A. So one needs the script mapping for the cal to kick in, huh? Thanks! –  morbusg Dec 11 '11 at 11:21
You have to know what's its code point: \cal A works in Plain TeX because \textfont2 has a calligraphic A in the slot corresponding to uppercase A. XITS Math has an A, there. –  egreg Dec 11 '11 at 11:30
Yeah I know. Unicode doesn't have a reserved range for calligraphic (which is distinctive from script style), and different fonts use different ways for cal. The Unicode range for script is that 0x1D49C..0x1D4CF (accounting for both upper- and lowercase). What I didn't know, was that I needed to use the script range for the stylistic set -switch to kick in. In Asana Math, for example, the calligraphic range isn't available via stylistic set 1 on script-range. –  morbusg Dec 11 '11 at 11:46
@morbusg: Mapping script to calligraphic seemed the most logical choice to me because of the similarity between both (they are even seen sometimes as mere stylistic variants of the same math alphabet rather than two different alphabets.) –  Khaled Hosny Dec 11 '11 at 13:06
@morbusg -- if you can find several math documents from "recognized" publishers that use calligraphic and script in contrast to one another, with different meanings, i can open the question of adding a calligraphic block with the unicode technical committee. when the symbol set for the stix project was being developed, i couldn't find such examples, which is why they aren't distinguished in unicode. everyone on the committee believed they were simply subjective alternates, either for "house style" or personal preference. it's a hard sell. –  barbara beeton Dec 11 '11 at 13:40