[XeTeX] Allowing Ucharcat to produce active characters

[Bruno Le Floch]

Hello,

I suggest allowing \Ucharcat to produce active characters.  See
three-line patch attached.  This would allow to produce active
characters expandably in all engines (pdfTeX, luaTeX, XeTeX, pTeX,
upTeX).  My code makes

    \Ucharcat `~ 13
    \expandafter\show\Ucharcat `~ 13
    \edef\foo{\expandafter\noexpand\Ucharcat `~ 13 }

run the code of the active ~ as if it had been typed directly, then show
its meaning, then do the equivalent of \def\foo{~}.

Bruno
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