AxesStack.add(((1, 1, 1), ()),..): Axes(0.125,0.1;0.775x0.8) vs Axes(0.125,0.1;0.775x0.8)                         ^key                ^ a                     ^a_existingmatplotlib/cbook.py", line 1410, in remove    raise ValueError('Unknown element o: %s vs %s' % (o, self._elements))Unknown element o: (((1, 1, 1), ()), <matplotlib.axes._subplots.AxesSubplot object at 0x7fd416508550>) vs [(((1, 1, 1), ()), (1, <matplotlib.axes._subplots.AxesSubplot object at 0x7fd416508550>))]    def add(self, key, a):        """        Add Axes *a*, with key *key*, to the stack, and return the stack.        If *a* is already on the stack, don't add it again, but        return *None*.        """        # All the error checking may be unnecessary; but this method        # is called so seldom that the overhead is negligible.        if not isinstance(a, Axes):            raise ValueError("second argument, %s, is not an Axes" % a)        try:            hash(key)        except TypeError:            raise ValueError("first argument, %s, is not a valid key" % key)        a_existing = self.get(key)        if a_existing is not None:HERE-->            Stack.remove(self, (key, a_existing))            warnings.warn(                "key %s already existed; Axes is being replaced" % key)            # I don't think the above should ever happen.        if a in self:            return None        self._ind += 1        return Stack.push(self, (key, (self._ind, a)))