code: def internal_files(self):
          
          ret_list = []
          for key, val in self.file_dict.items():
              # For internal files we only want files that were marked as
              # internal
              if val & FileFlags.internal_mask:
                  ret_list.append(key)
          return ret_list
docs: Return a list of the intermediate files produced by this link.
  
          This returns all files that were explicitly marked as internal files.