code: def issubset(self, other):
          
          other = self._cast_to_frameset(other)
          if other is NotImplemented:
              return NotImplemented
          return self.items <= other.items
docs: Check if the contents of `self` is a subset of the contents of
          `other.`
  
          Args:
              other (:class:`FrameSet`):
  
          Returns:
              bool:
              :class:`NotImplemented`: if `other` fails to convert to a :class:`FrameSet`