Remove the first occurrence of *value*.  If *value* is not a member,
        does nothing.

def discard(self, value):
        """
        Remove the first occurrence of *value*.  If *value* is not a member,
        does nothing.
        """
        _set = self._set
        if value in _set:
            _set.remove(value)
            self._list.discard(value)