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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 default(self, node):
 """Augment write default routine to record line number changes"""

        if hasattr(node, 'linestart'):
            if node.linestart:
                self.source_linemap[self.current_line_number] = node.linestart
        return super(LineMapWalker, self).default(node)