[XeTeX] Modification of xepersian-mathsdigitspec and parsidigits

[nathan.sidoli at utoronto.ca]

How do I modify the behavior of xepersian-mathsdigitspec and  
parsidigits so that the characters for 4, 5, and 6 correspond to the  
Arabic numerals (U+6064,U+6065,U+6066), as opposed to Persian numerals  
(U+60F4,U+60F5,U+60F6)? I found a parsidigits.map file online (by  
Vafa), which has the mapping and would be easy to modify, but I cannot  
find this file anywhere on my system. The file  
xepersian-mathsdigitspec seems to refer to parsidigits and I can't see  
how to make the necessary changes in a modified version of this file.