[XeTeX] detection of absent characters within a font

[Adam Lindsay]

Hi all,

As basic ConTeXt/XeTeX support is mostly under control now, I was looking
to enable some "extras".

The first was detecting whether or not a character is present in the
current font. I tried checking the width of a scratchbox with the
character, but it *doesn't* have zero width when the font is missing that
character, in my tests this morning. (No, for this application, I'm not
too fussed about combining diacriticals.)

Widths didn't work. Is there any other indicator that a character is
missing in action?
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 Adam T. Lindsay, Computing Dept.     atl at comp.lancs.ac.uk
 Lancaster University, InfoLab21        +44(0)1524/510.514
 Lancaster, LA1 4WA, UK             Fax:+44(0)1524/510.492
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