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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 format_key_from_backend(self, key):
 """Strip the configured prefix from a key retrieved from the backend before passing it on to other pghoard code and presenting it to the user."""

        if not self.prefix:
            return key
        if not key.startswith(self.prefix):
            raise StorageError("Key {!r} does not start with expected prefix {!r}".format(key, self.prefix))
        return key[len(self.prefix):]