code: def get_previous_identifiers(self):
          
          previous_identifiers = None
          stack = self.get_run_as_identifiers_stack()  # TBD:  must confirm logic
  
          if stack:
              if (len(stack) == 1):
                  previous_identifiers = self.identifiers
              else:
                  # always get the one behind the current
                  previous_identifiers = stack[1]
          return previous_identifiers
docs: :returns: SimpleIdentifierCollection