[XeTeX] velthuis tranliteration scheme and virama

[François Patte]

Bonjour,

I am facing a problem with virama while using the velthuis 
tranliteration scheme : according the velthuis-sanskrit.map file, virama 
is mapped to "&" and "_".

I use FreeSerif fonts and some ligatures can be acceeded using a virama 
eg.:

r+r-vocalic (r.r) gives: रृ or
r+virama+r-vocalic (r&.r) gives: र्ऋ

To get the second ligature (र्ऋ), which I prefer, I have to change the 
catcode of "&" or"_":

\catcode`\&=11\relax or \catcode`\_=11\relax

Both cases have one major disadvantage: for "&", I can't use tabular 
environments, for "_"; I can't use mathematical mode...

Is there a way around this problem? Is there a feature in fontspec 
allowing the use of alternative ligatures?

Or do I have to map the virama to an other character (which one?) in 
velthuis-sanskrit.map file?

Thank you.

Regards
-- 
François Patte
UFR de mathématiques et informatique
Laboratoire CNRS MAP5, UMR 8145
Université Paris Descartes
45, rue des Saints Pères
F-75270 Paris Cedex 06
Tél. +33 (0)6 7892 5822
http://www.math-info.univ-paris5.fr/~patte