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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def focus_next_sibling(self):
 """focus next sibling of currently focussed message in thread tree"""

        mid = self.get_selected_mid()
        newpos = self._tree.next_sibling_position(mid)
        if newpos is not None:
            newpos = self._sanitize_position((newpos,))
            self.body.set_focus(newpos)