Question About XDOXSLT Set Variable

JD Edwards EnterpriseOne Release E920

Oracle BI Publisher Desktop 12.2.1.4.0

Build 12.214.00.000

I am having some difficulty learning to use the XDOXSLT: set_variable.

I would like for the variable TotDue to include the sum of (Current_Due + Past_30_Due + Past_60_Due + Past_90_Due). The code below is what I currently have which only includes the summation of Current_Due. If someone could show me the syntax to include Past_30_Due, Past_60_Due and Past_90_Due, I would greatly appreciate it.

  <?for-each:current-group()?>

   <?Customer_Number?>

   <?Invoice_Number?>

   <?Current_Due?>

   <?Past_30_Due?>

   <?Past_60_Due?>

   <?Past_90_Due?>

   <?xdoxslt: set_variable($_XDOCTX, 'TotDue', xdoxslt:get_variable($_XDOCTX, 'TotDue') + Current_Due)?>