code: def get_apis(self):
          
          out = set(x.api for x in self.types.values() if x.api)
          for ft in self.features.values():
              out.update(ft.get_apis())
          for ext in self.extensions.values():
              out.update(ext.get_apis())
          return out
docs: Returns set of api names referenced in this Registry
  
          :return: set of api name strings