class_docstr: Checks whether a slot is set at the current step.
  
  The actual `SlotSet` event might have happened at an earlier step.
class_docstr_tok: ["Checks","whether","a","slot","is","set","at","the","current","step",".","The","actual","`","SlotSet","`","event","might","have","happened","at","an","earlier","step","."]
code: def _non_negated_version_applies_at(self, event: Event) -> bool:
      if isinstance(event, SlotSet) and event.key == self.text:
          # slot is set if and only if it's value is not `None`
          return event.value is not None
      if self.history:
          was_set = (
              self.history[-1]
              if not self.negated
              else not self.history[-1]
          )
          return was_set
      return False
  
imports: from rasa.shared.core.domain import Domain
  from typing import Optional, Text, List
  from rasa.core.evaluation.marker_base import OperatorMarker, ConditionMarker, MarkerRegistry, Marker
  from rasa.shared.core.events import ActionExecuted, SlotSet, UserUttered, Event
  import logging
label_desc: Use image node_id 4 for calling the SlotSetMarker obj's underlying member method code with example usage: obj._non_negated_version_applies_at(event) and returns: False, unknown, was_set