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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 get_mark(self):
 """Gets an expanded, collapsed, or leaf icon."""

        if self.is_leaf:
            char = self.icon_chars[2]
        else:
            char = self.icon_chars[int(self.expanded)]
        return urwid.SelectableIcon(('mark', char), 0)