Called when a card is removed.
        Removes the card from the tree.

def OnRemoveCards(self, removedcards):
        """Called when a card is removed.
        Removes the card from the tree."""
        self.mutex.acquire()
        try:
            parentnode = self.root
            for cardtoremove in removedcards:
                (childReader, cookie) = self.GetFirstChild(parentnode)
                found = False
                while childReader.IsOk() and not found:
                    if self.GetItemText(childReader) == \
                            str(cardtoremove.reader):
                        (childCard, cookie2) = self.GetFirstChild(childReader)
                        self.SetItemText(childCard, 'no card inserted')
                        found = True
                    else:
                        (childReader, cookie) = \
                            self.GetNextChild(parentnode, cookie)
            self.Expand(self.root)
        finally:
            self.mutex.release()
        self.EnsureVisible(self.root)
        self.Repaint()