Concatenate properties

[Tyler Grinn]

I'm exporting sub-trees as pdf files for some classes I'm taking:

# -*- org-use-property-inheritance: t; -*-

* Class A
  :PROPERTIES:
  :EXPORT_FILE_NAME: support/Class A
  :END:
** Assignment 1
   :PROPERTIES:
   :EXPORT_FILE_NAME+: /assignment-1
   :END:
   Some assignment for Class A
** Assignment 2
   :PROPERTIES:
   :EXPORT_FILE_NAME+: /assignment-2
   :END:
   Some other assignment for Class A
* Class B
  :PROPERTIES
  :EXPORT_FILE_NAME: support/Class B
  :END:
** Assignment 1
   :PROPERTIES:
   :EXPORT_FILE_NAME+: /assignment-1
   :END:
   Some assignment for Class B
** Assignment 2
   :PROPERTIES:
   :EXPORT_FILE_NAME+: /assignment-2
   :END:
   Some other assignment for Class B
   
And this works great, except there's always a space between
'support/Class A' and '/assignment-1.pdf'. Is there any way to
concatenate the two properties rather than join them with spaces?

Best,

Tyler