[XeTeX] Greek (and other Unicode) letters in math mode

[Jean-Louis Cordonnier]

I am using the "Compose Key" with Kubuntu 10.10.
http://en.wikipedia.org/wiki/Compose_key

Compose + space + a → α

Compose + space + b → β

Compose + space + g → γ

Compose + space + G → Γ

Adding in the configuration file is possible, for instance to get :

Compose + К (cyrillic K) + , → Ӄ (tchoukche letter = K with hook)
http://en.wikipedia.org/wiki/Ka_with_hook

etc.