[XeTeX] Fallback for "Missing character: There is no _ in font"?

[Oleg Parashchenko]

Hello,

for my documents I use a font "Foobar Cyrillic". Time to time, the text
contains greek letters. Instead of this letters I get an empty space in
PDF and the message "Missing character: There is no ? in font xxx" in log.

Is it possible to configure Xe[La]TeX to use fallback fonts? For example,
if a glyph isn't found in "Foobar Cyrillic", then look for it in "Foobar
Greek", then in "Foobar East European", then ...., "Super STIX".

Thanks!

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