Αλγοριθμος επαληθευσης ΑΦΜ σε XPath

In order to validate the check digit of the Greek Tax ID(ΑΦΜ) using XPath expression is much more concise compared to other languages:

string-length(.) = 9 and
substring(., 9,1) = (
  substring(., 1,1) * 256 + 
  substring(., 2,1) * 128 + 
  substring(., 3,1) * 64 + 
  substring(., 4,1) * 32 + 
  substring(., 5,1) * 16 + 
  substring(., 6,1) * 8 + 
  substring(., 7,1) * 4 + 
  substring(., 8,1) * 2
) mod 11 mod 10

This expressions is useful for XForms applications.