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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_root( self, key ):
 """Retrieve the given root by type-key"""

        if key not in self.roots:
            root,self.rows = load( self.filename, include_interpreter = self.include_interpreter )
            self.roots[key] = root
        return self.roots[key]