XPath access constraint on attribute of CurrentUser

Assume the following model: Company with a 1-n relation to Employee Account (specialisation from System.User) with a 1-1 to Company.   So our accounts are linked to a Company. When a user logs in it should only be able to edit Employees from their own company. I want to add a Xpath access constraint on the Employee entity which would be something like --> [ Employee_Company = '[%CurrentUser%]' /Account_Company] But I'm not sure how to achieve this.  
1 answers

The chain to constrain by should be Employee_Company -> Company -> Account_Company = currentUser, so something like:

[ Employee_Company/Company/Account_Company = '[%CurrentUser%]' ]