code: def get_contained_extras(marker):
      
      if not marker:
          return set()
      marker = Marker(str(marker))
      extras = set()
      _markers_collect_extras(marker._markers, extras)
      return extras
docs: Collect "extra == ..." operands from a marker.
  
      Returns a list of str. Each str is a speficied extra in this marker.