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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 filename( self, node ):
 """Extension to squaremap api to provide "what file is this" information"""

        if not node.directory:
            # TODO: any cases other than built-ins?
            return None
        if node.filename == '~':
            # TODO: look up C/Cython/whatever source???
            return None
        return os.path.join(node.directory, node.filename)