code: def append(self, action):
          ''' Add a undoable to the stack, using ``receiver.append()``. '''
          if self._receiver is not None:
              self._receiver.append(action)
          if self._receiver is self._undos:
              self._redos.clear()
              self.docallback(f append(self, action):
          ''' Add a undoable to the stack, using ``receiver.append()``. '''
          if self._receiver is not None:
              self._receiver.append(action)
          if self._receiver is self._undos:
              self._redos.clear()
              self.docallback()
docs: Add a undoable to the stack, using ``receiver.append()``.