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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 add(self, item):
 """ Add a file to the manifest. :param item: The pathname to add. This can be relative to the base. """

        if not item.startswith(self.prefix):
            item = os.path.join(self.base, item)
        self.files.add(os.path.normpath(item))