code: private boolean canCompare(CheckBox check1, CheckBox check2) {
  
          return !((check1 == null) || (check2 == null) || (check1.getData() == check2.getData()));
      }
docs: Checks if two different versions are selected, and so can be compared.<p>
  
  @param check1 the first selected check box
  @param check2 the second selected check box
  
  @return true if the check boxes correspond to two different versions