docs: def filtered(f):
      '''
      Decorator function that wraps functions returning pandas
      dataframes, such that the dataframe is filtered
      according to left and right bounds set.
      '''
  
      def _filter(f, self, *args, **kwargs):
          frame = f(self, *args, **kwargs)
          ret = type(self)(frame)
          ret._lbound = self._lbound
          ret._rbound = self._rbound
          return ret
  
      if HAS_DECORATOR:
          return decorator(_filter, f)
      else:
          def err_func(*args, **kwargs):
              raise RuntimeError("`pip install decorator` required")
          return err_func