code: def audit_trail_users(self):
          '''A set of all usernames recorded in the :attr:`audit_trail`,
          if available.'''
          if self.audit_trail:
              return set([r.user for r in self.audit_trail.records])
          return set(f audit_trail_users(self):
          '''A set of all usernames recorded in the :attr:`audit_trail`,
          if available.'''
          if self.audit_trail:
              return set([r.user for r in self.audit_trail.records])
          return set()
docs: A set of all usernames recorded in the :attr:`audit_trail`,
          if available.