[GRASSLIST:362] ps.map: subscript in comments?

Does anyone know how to get ps.map's comment command to deal with subscripts?

The usual X_{a} does not seem to work here.

... test.ps ...
(Helvetica-Bold) FN 13.0 SF
(Figure 2\(a\). Subscripts like X_{a} do not work here)
72.0 78.7 MS
...

thanks
Hamish