code: def retract(self, idx_or_declared_fact):
          
          self.facts.retract(idx_or_declared_fact)
  
          if not self.running:
              added, removed = self.get_activations()
              self.strategy.update_agenda(self.agenda, added, removed)
docs: Retracts a specific fact, using its index
  
          .. note::
              This updates the agenda